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Tool Steel Selection

Z-Series PM Tool Steel Selection

Tool Steel Selection_pdf

PROPER TOOL STEEL SELECTION is a critical component in maximizing productivity and, thus, maximizing profits. With today’s more highly alloyed and engineered work materials, the demands on tooling have increased significantly, pushing the limits for key performance properties including wear resistance, impact resistance, compressive strength and coating technology.

Zapp Tooling Alloys’ Z-Series PM family of tool steels is produced through a particle metallurgy process that offers a better solution for the most demanding tooling applications. In applications requiring extreme toughness, high wear resistance, and/or compressive strength, Zapp offers Engineered Solutions.

BENEFITS OF ZAPP Z-SERIES PM STEELS The Particle Metallurgy process was developed to address key weaknesses of tool steel. Traditional ingot casting of highly alloyed tool steels has severe toughness challenges, caused by an undesirable microstructure. PM steels achieve a uniform microstructure, which dramatically increases toughness and allows for greater wear resistance.

Z-TUFF PM®

Designed specifically for high toughness applications in combination with good compressive strength.

■ Typical application hardness of Rc 60-62. Attainable hardness of Rc 62 max.

■ Impact resistance levels of traditional shock resistant tool steels (AISI S7).

 

Z-WEAR PM®

Optimizes the combination of impact toughness, wear resistance and enhanced compressive strength.

■ Enhanced compressive strength with working hardness capability to Rc 64 max.

■ Higher impact resistance than AISI A2, D2, M2 and PM M4.

 

Z-M4 PM®

The standard traditional workhorse PM grade for cold work applications offering a good combination of impact and wear resistance with good compressive strength.

■ Good compressive strength with working hardness of Rc 64 max.

■ Higher impact resistance than A2, D2 and M2 but lower toughness than Z-Wear PM.

 

Z-A11 PM®

Designed for applications requiring high wear resistance with reasonable toughness capabilities.

■ Good compressive strength with working hardness of Rc 63 max. Typically used at Rc 60-62.

■ Excellent wear resistance with a carbide volume of 17.5%.

■ Expected wear improvement of approximately two times the performance of PM M4.

■ Toughness/impact resistance in the range of conventionally produced D2 and M2 at similar hardnesses.

 

Z-MAX PM®

A Cobalt bearing PM grade designed for cold work applications requiring high wear resistance in combination with excellent compressive strength.

■ Superior compressive strength with a standard cold work application hardness of Rc 64-66.

■ Excellent wear resistance with a carbide volume of 19.5%.

■ Wear resistance exceeds that of A11 due to higher carbide volume and attainable working hardness.

■ Cobalt content (9%) to enhance and strengthen material matrix.

 

Engineered Solutions

SERIES I: Introducing the Concept of Tool Steel Microstructure

A basic tenant of metallurgy is that the properties of a metal are a function of its microstructure. Consequently, further understanding of tool steel requires an appreciation for the type of microstructure offered by each grade. This may seem a bit abstract to some, but is not difficult even from a layman’s point of view. Microstructure tells a more complete story in that it reflects not only the composition of the grade, but also the way the material was manufactured and the thermal history of the article in question.

The constituents of tool steel microstructure primarily involve alloy carbide in a hardened martensitic matrix. The type, size, and distribution of the carbides largely determine the wear and toughness characteristics of each particular grade. This attributes are tied to the method for melting the alloy, as well as, the amount of hot working involved in the production process. Conventional melt processes utilize ingot casting of materials which leads to non-uniform structures. As cost pressures continue, mill producers have introduced larger ingots into the process to reduce overall costs which can be detrimental to the tool steel structure. Particle metallurgy (PM) melt practices involved the atomization of metals which leads to uniform carbide distribution and structure.

Differences can be readily seen when examining cold work grades such as Z-Wear PM and conventionally produced D2 (figure 1). While both grades can offer similar wear performance, the large, banded carbides of the D2 result in potential for cracking and a greater tendency to chip in finished tools. The Z-Wear PM is produced by particle metallurgy methods which result in a microstructure with fine uniformly distributed chrome and vanadium carbide. This provides better toughness and higher hardness capability which together facilitate more versatile and consistent tool performance.

PM Microstructures

Within the Zapp Tooling Alloys series of PM alloys, grades are differentiated by the type and volume of carbide that is present in the heat treated microstructure. The Z-Wear PM offers the highest toughness and impact strength while maintaining good wear performance at a typical working hardness in the range of RC 58-62. The Z-M4 PM high speed steel offers increased wear resistance and attainable hardness (up to RC 64-66) while maintaining an intermediate level toughness. The Z-A11 PM has a high volume of vanadium carbide which offers exceptional wear and abrasion resistance (at RC 58&-62) while toughness will be somewhat less by comparison with the other 2 grades (figure 2).

Microstructure-Cold work Applications
Tool Steel Selection on the Basic of Microstructure-Cold work Applications.

The difference in the microstructures of these grades allows them to cover a broad range of application requirements. Depending on factors such as type of part material, stock thickness, die design, desired tool life, etc. it should be possible to find a grade which fits the bill. The uniform microstructure of the PM grades can provide significant benefits in regard to predictable and consistent tool performance.

Gary Maddock, Technical Manager, Zapp Tooling Alloys, Inc. gmaddock(at)zapp.com

The “Tool Steel Training” Series is meant as an educational and technical tool for Zapp’s customer and will be a regular part of the zappnews.com marketing efforts. Look for future discussing other areas of interest relating to tooling performance improvements. If you have a subject suggestion, please feel free to submit a request to Gary Maddock at gmaddock(at)zapp.com or Harry O’Brien at hobrien(at)zapp.com.

SERIES II: Typical Failure Modes for Cold Work Tooling and Their Association with Microstructure

The first installment of our technical discussion series introduced the concept of microstructure and how it relates to material properties. In the case of tool steels, examining microstructure is also a key step in understanding specifically how and why tools fail.

conventional-aisis-m2-heat-treated-microstructures
Conventional Aisis M2 Heat Treated Microstructures

The first photomicrograph (Figure 1) shows a 500X view of the microstructure of a forming punch made from 3" diameter M2 material which failed prematurely due to longitudinal cracking. The microstructure appears normal for standard quality M2 and it also indicates that the heat treatment process was correct. However, it is clearly evident how the crack follows along a line of banded carbides. The carbides provide wear resistance needed in heavy duty punch applications, but the banding is a natural occurrence resulting from the solidification process utilized in the production of conventionally melted high speed steel ingots. It is more pronounced in larger diameter bars and has the unintended consequence of providing a very convenient path for crack propagation under the right combination of stress and impact. The banding is even more evident when looking at the structure with lower magnification as shown in the second photomicrograph taken at 100X (Figure 2).

Microstructure
Figure 3 (500X)

The third photomicrograph (Figure 3) shows a 500X view taken of another punch used in the same application. This punch was not found to exhibit cracking, but unfortunately it developed pronounced issues with corner chipping. The particular bar of material appears to show less carbide banding. However, examination of the microstructure revealed many large clusters of carbide. This again is a result of the solidification process, and the photo shows how the chipping can subsequently originate at the large carbides. When a carbide cluster occurs near the edge of the punch face, it provides a ready means for premature tool failure.

In both examples, the tooling did not have the opportunity to “wear out.” The punches had to be changed early resulting in unplanned down time and added cost. Both problems could be solved by the use of PM tool steels. One alternative would be to consider the use of Z-Wear PM. This grade offers less wear resistance than the M2 due to the lower actual carbide volume. However, the lower alloy content combined with the uniform PM structure would offer over 3 times the toughness of the conventional grade. This would almost certainly resolve the cracking and/or chipping issues and shift the punch performance into a more predictable wear scenario. One could also consider the use of Z M4 PM which is a higher alloy, higher cost grade. While not offering as much of an improvement in toughness (compared to Z-Wear), it would provide wear resistance that in most cases exceeds that of the conventional M2. Both grades are valid options which could improve performance and lower overall costs. A typical PM microstructure is pictured below.

PM Microstructure

Questions or comments may be sent to Gary Maddock at gmaddock(at)zapp.com.

SERIES III: Basics of Heat Treatment • Part 1

It is the heat treat process that essentially “pulls the trigger” to unlock the strength‚ wear resistance‚ and toughness capabilities of the alloy being treated. Yet despite the well recognized importance, heat treat related issues remain commonplace much because of the difficulty involved in determining if the job was done correctly (unless something REALLY bad happened‚ a poorly heat treated tool looks the same as a good one). We rely day to day on the good old hardness test as the simple to perform measurement by which we gage the success of the heat treat operation. Unfortunately‚ the hardness test just doesn’t tell the whole story. It is not unlike attempting to build a die to extremely close tolerances with nothing but a yard stick to measure your work.

From a metallurgical perspective‚ the heat treat process actually involves a number of specific and fairly complex changes in the metallurgical structure of the materials. This is where we talk about topics like austenite transforming to martensite‚ etc. These changes can be easily observed under a microscope using a polished and etched sample which makes analysis of microstructure the best method by which to judge the outcome of the heat treat process. Consider the following example as a case in point.

These photos were taken from a pair Z–Wear PM fine blanking punches which both checked RC 60–62‚ but were found to exhibit microstructures which appear very different. The sample shown in figure 1 came from a punch which failed (cracked) prematurely after only 6000 hits, and a number of problems are clearly evident. The pronounced grain boundaries indicate the tool had been overheated (2100°F) during hardening and was poorly quenched. The light etching matrix (background) indicates the tool was also poorly tempered and contains retained austenite and untempered martensite. These factors together resulted in a near total loss of toughness. The structure shown in figure 2 came from a new tool which was correctly hardened at 1950°F‚ 6 bar quenched‚ and triple tempered. It went on to run 25‚000 hits. Remember that both punches checked at the specified hardness of Rc 60–62.

Full coverage of all the ins and outs concerning the proper heat treatment of tool steel obviously is beyond the scope of this short article. However, future installments will continue to take a look at some of the common issues and will attempt to answer some of the most frequently asked questions concerning the heat treatment of the Zapp Tooling Alloys line of PM grades. Along the way we will also provide some practical advice in regard to working with your heat treat source and how heat treatment can be used as a means to optimize the performance of high alloy tool materials.

Subsequent releases from Zapp’s heat treatment series will be released shortly and address: 1) Austenitizing Temperatures and Quench Rates, 2) Proper Tempering Procedures, and 3) Cryogenic Treatments.

Questions or comments may be sent to Gary Maddock at gmaddock(at)zapp.com.

SERIES III: Basics of Heat Treatment • Part 2

Tool steels are heat treatable because of the fact that iron has several allotropic forms (don’t panic — this gets pretty basic!). This has to do with the way the iron atoms arrange themselves in the metallic crystal structure. At room temperature‚ iron usually exists as a body centered cubic (bcc) arrangement of atoms known as alpha (α) iron (or ferrite). When heated, iron transforms to a face centered cubic arrangement of atoms known as gamma (γ) iron (or austenite). These low and elevated temperature forms of iron are shown in figure 1.

basics of heat treatment
Figure 1

The first step in the heat treatment of tool steels (which are alloys of iron‚ carbon and various other elements) involves heating beyond the temperature at which structure transforms to austenite (austenitizing temperature). Carbon plays an extremely critical role in this process. It is insoluble in ferrite and exists as part of a separate carbide phase in the annealed (or soft) condition. However‚ carbide will actually start to dissolve once the material becomes austenitized at high temperature. This is a reversible process and no hardening will occur provided the material is cooled slowly back to room temperature (austenite will transform back to ferrite plus carbide). However‚ rapid cooling (or quenching) from high temperature will result in iron atoms becoming forced into a new‚ highly distorted arrangement known as martensite. Fast cooling does not allow sufficient time for diffusion to occur and the dissolved carbon atoms essentially find themselves trapped between iron atoms which are attempting to transform back to the bcc structure. The crystal lattice becomes highly stressed in this condition resulting in a significant increase in hardness and strength of the material. Martensite must subsequently be tempered which helps to reduce brittleness while improving toughness and shock resistance.

Obviously there is much detail that has been left out of the discussion at this point‚ and it could be ascertained that the devil is in fact in the details. However‚ the above description provides a framework from which to examine a schematic of a typical tool steel harden and temper cycle as shown in figure 2. This depicts the series of heating‚ soaking‚ and cooling steps that accompany the basic microstructure transformations. The essential parts of the process can be broken down into preheating‚ high heat (or austenitizing)‚ quenching‚ and then the various intervals of heating and cooling that are involved in tempering (the schematic shows a double temper).

Schematic of heat treatment cycle
Figure 2: Schematic of heat treatment cycle

It is relatively easy these days to program set points into the instrumentation of a modern heat treat equipment. However‚ the results ultimately depend on the actual time and temperature experienced by the parts being run, and herein lies much of the “art” involved in the process. Figure 3 shows real life data taken from thermocouples attached to a large cutting tool during salt bath hardening which provides an interesting comparison to the generic schematic shown above.

Figure 3

The information presented in this installment is intended to provide a foundation for the ongoing “Tool Steel Heat Treatment” series. The next installment will begin to take a closer look at the individual parts of the process and will also include microstructure comparisons of samples in the annealed‚ as quenched‚ and tempered condition.

Questions or comments may be sent to Gary Maddock at gmaddock(at)zapp.com.

SERIES III: Basics of Heat Treatment • Part 3

Tool steels are generally supplied in the fully annealed condition. In the annealed (soft) condition, the material has the best machinability which facilitates the “soft” tool making operations such as cutting, drilling and milling. The hardness of annealed tool steels varies by grade but typically runs in the range of 229 to 285 BHN with low alloyed steel grades at the lower annealed hardness range and higher alloyed grades at the upper end of the range.

Figure 1 below shows the microstructure of annealed Z-Wear PM material. You can be observe how its ferritic matrix contains a large number of both large (primary) and small (secondary) carbides. These appear as large and small globular shapes outlined in dark gray. In fact most of the alloy and carbon are in the form of carbide when the grade is in this condition.

Schematic of heat treatment cycle
Figure 1: Annealed Z-Wear PM (ferrite and carbide)
Annealed Z-Wear PM
Figure 1: Annealed Z-Wear PM (ferrite and carbide)

Let’s take a step by step look at the process of transforming the soft, annealed tool steel into hardened and tempered material ready to be finish ground and put into application. Each step in this process has a specific purpose, and the final outcome (a properly hardened tool with an appropriate hardness and tempered microstructure) is dependent on the correct execution of each individual step.

STEP 1. PREHEATING

In general, the heat treatment of tool steels involves a batch process performed in either a vacuum furnace or salt bath. After fixturing and loading into baskets, the tools are ramped up to a preheat set point. This is typically done at around 1500 to 1600°F which is near to the point where the material transforms from the ferrite and carbide structure to austenite. Preheating minimizes the surface to center temperature differential for uniform heating and, therefore, reduces residual stresses to minimize distortion in tools. Furnaces with programmable controls may be set to have multiple preheats which further aids in uniform heating, as well as, monitoring time and temperature control throughout the load.

STEP 2. AUSTENITIZING TEMPERATURE (HIGH HEAT)

During austentizing, the material arrives at the prescribed high heat or hardening temperature. Austenitizing is a critical step in the heat treatment process. It is critical to achieve both the correct temperature level and the necessary time at this temperature. At this point, the tool steel is fully austentitized and begins to dissolve into solution the necessary amount of carbon and alloy to subsequently harden. Insufficient time and temperature can result in low hardness while excessive time and temperature will result in serious damage to the material due to overheating. The more simple tool steels have relatively straight forward recipes for hardening while the high speed grades and higher alloy PM grades require a bit of a selection process to determine the optimum parameters given the desired hardness. The heat treatment of the PM grades can actually be tailored to suit specific applications.

STEP 3. QUENCHING

If austenitizing is akin to putting the “bullets into the gun”, then it is quenching that “pulls the trigger” on the hardening process. Quenching is the process of rapidly cooling from the high heat such that the material does not have time to return to its original soft, annealed condition. Because the carbon and alloy dissolved at high temperature essentially become “trapped”, the quenched material is forced into a martensitic matrix which has very high hardness and strength. The actual transformation of the material from austenite to martensite begins at around 400 to 500°F and continues progressively as temperature is further decreased. The ideal quench rate is dependent upon the composition of the tool steel. Some tool steels require oil quenching or even water quenching, but the most common alloys today are said to be air hardening. In any case, strong positive cooling is critical to the development of an optimum heat treated structure.

Figure 2 shows the structure of Z-Wear PM in the as-quenched condition after vacuum hardening from an austenitizing temperature of 1900°F. In comparison to the annealed structure in Figure 1, most of the small carbides are no longer visible because these carbides were dissolved and have been trapped into solution. Figure 2 also shows some of the austenite grain boundaries with fine spacing which is an indication of proper hardening and good tool steel quality.

As-Quenched Z-Wear PM
Figure 2: As-Quenched Z-Wear PM (martensite plus retained austenite)

STEP 4. TEMPERING

The final steps in the heat treat process are tempering. Although not as complex as hardening, tempering is equally critical. Tempering specifically improves as-quenched structure which is highly stressed and inherently brittle. During tempering, the material is heated to an intermediate temperature which is grade and hardness dependent. This temperature is typically in the range of 400 to 1100°F and held for several hours before cooling back to room temperature. The goal of tempering is to arrive at a structure that consists entirely of well tempered martensite (tougher structure). This sounds simple, but can involve challenges especially in regard to the high alloy tool steels and high speed grades. High alloy tool steels retain a percentage of the austenite after quenching which can subsequently transform to untempered martensite during the temper process. As a result, these alloy types require multiple tempers (sometimes referred to as having a double or triple draw) in order to achieve a stable, fully tempered structure.

Figure 3 show the microstructures of hardened Z-Wear PM following a single and double temper, respectively, at 975°F. The degree of darkening within the matrix provides a good indicator of the progress made due to the fact that the specimen will more readily etch in the tempered condition (while as-quenched structure does not etch much at all). The image on the right reflects a proper final heat treated structure.

Tempered Z-Wear PM
Figure 3: Tempered Z-Wear PM

This series has thus far provided an overview of the changes that occur during the heat treatment of tool steels, as well as, why each step is critical to the final performance in tooling applications. Future series installments will take a more thorough look at each of the heat treat steps. Along the way we will cover some of the equipment considerations, provide a few hints and tips, and also consider some of the areas that can go wrong.

Questions or comments may be sent to Gary Maddock at gmaddock(at)zapp.com.

SERIES III: Basics of Heat Treatment • Part 4

The first three issues of Zapp’s tool steel heat treatment series have examined the process primarily from a metallurgical point of view. We have shown how the heat treat process can be described in terms of the metallurgical transformations that take place relative to the microstructure of the material. We can now begin to look at the process in further detail by considering the practical aspects of what it takes to get the job done precisely in the commercial realm.

In theory, one could harden tool steel with a torch and a bucket of oil (and some might try in a pinch). However, unless performed by a magician, results by this method will no doubt be disappointing. Although sometimes viewed as a black art, the science of heat treating actually involves following carefully controlled heating and cooling steps in order to accomplish specific transformations within the material. Understanding and controlling the heat treatment process are paramount. In this scenario, the critical process variables consist of the following:

  • Heating (ramp) rates
  • Time and temperature of the parts at critical set points
  • Cooling (quench) rates
  • Atmosphere for surface protection

The importance of the thermal processing variables (items 1 thru 3) stands to reason (much like baking a cake). However, the need for atmosphere control needs some further discussion. This is critical because oxidation, scaling, and decarburization can occur if the work piece is exposed to air while at elevated temperature. The result is not only a rough open surface, but also a loss of hardness (decarburization) requiring significant grinding to remove the decarb layer to restore the proper surface hardness and integrity of the tooling. Evidence of decarb and scaling are readily apparent when looking at a microstructure as shown in Figure 1. In this type of view, one can directly measure the amount of removal that would be needed to get down to normal structure.

Basics of Heat Treatment
Example of decarburization in a P/M M4 H.S.S. Sample was held in an air furnace at 2.250°F for 5 minutes, oil quenched, and tempered at 1.050°F for 2 hours + 2 hours.

Etchant: 2% Nital
Original Magnification: As-indicated

The degree of difficulty involved in the heat treatment of tool steels is very much related to the alloy content of the grade in question. Higher alloy grades tend to be much less forgiving, and require strict process control that can become more involved as demonstrated by the furnace chart shown in figure 2. Figure 2 is also an example of how a modern vacuum furnace can be utilized to manage the complete hardening cycle including vacuum/pressure, ramp rates, set points, and quench rates. Output from a load thermocouple was used to guarantee proper control of actual part time and temperature. The cycle consists of 4 preheat steps, a high heat soak, and a high speed quench including an isothermal hold to minimize distortion.

Furnace Chart for Processing High Alloy Tool Steel
Figure 2: Furnace Chart for Processing High Alloy Tool Steel

An example of a typical load that has just been run through such a hardening cycle is shown in Figure 3. It is apparent that the parts are very clean with little in the way of appearance to indicate they have been hardened. The picture also reveals how the parts were fixtured to allow uniform heating and cooling. It is also apparent how a load thermocouple in a “dummy” slug was used to maintain accurate control of the actual load temperature.

Load of HSS Tool Blanks after Hardening
Figure 3: Load of HSS Tool Blanks after Hardening

Up to this point we have established the need for process control and have identified the elements that must be given consideration if the job is to be done correctly. Vacuum processing has in fact become the most common way of treating tool steels, but salt baths continue to play an important if somewhat limited role. There also continues to be examples of very basic “tool room” type processing that can be found in everyday use. In the next installment, we will continue looking at the equipment used and compare the pros and cons of the methodology required in each case.

Questions or comments may be sent to Gary Maddock at gmaddock(at)zapp.com.

SERIES III: Basics of Heat Treatment • Part 5

Current Trends — Vacuum Processing

Modern 6 Bar Vacuum Furnace Installation
Figure 1: Modern 6 Bar Vacuum Furnace Installation

Much has been discussed in previous parts of this series relative to vacuum heat treatment which has become the most prevalent method for heat treating tool steels. An example of a state of the art installation at Gleason Cutting Tools is shown in figure 1. A summary of pros and cons relative to this method would include the following:

Vacuum Heat Treatment Advantages

  1. Absence of atmosphere keeps tools clean with excellent surface integrity
  2. Allows accurate and uniform heating/cooling
  3. Digital controls capable of advanced process management (and documentation).
  4. Provides consistent results provided proper parameters are employed.
  5. Good control of distortion is possible depending on the part configuration and fixturing methods used.

Vacuum Heat Treatment Disadvantages

  1. High capital cost.
  2. Batch type process can make it sometimes difficult to accommodate special cycles and temperatures.
  3. Relatively high operational and maintenance costs.
  4. Not all vacuum furnaces are created equal... older equipment can be less effective compared to the latest technology.
  5. Cycle time can be relatively long (4–6 hours for hardening depending on load), and lead times can become extended due to batching requirements.

One matter relative to vacuum heat treating that needs further mention is the issue of quench rate. Vacuum furnaces quench by backfilling with an inert gas such as nitrogen which is then recirculated under force through the load and an attached heat exchanger. This is roughly equivalent to an air cool and was one of the limitations of the earlier furnaces. The higher alloy grades and thicker section tools can benefit tremendously from a faster quenching which can now be accomplished in the newer furnaces which quench using high pressure gas. The nomenclature used to describe this equipment refers to the quench pressure capability such as 6 bar (six times atmospheric pressure) etc. A 6 bar furnace will have a quench rate nearly double a 2 bar furnace which is nearly double that of a negative pressure quench unit.

Comparison of as-quenched microstructures for cold work die steel
Figure 2: Comparison of as-quenched microstructures for cold work die steel with 2 bar quench of left versus 6 bar quench at right (both RC 60).

Figure 2 shows the effect of quench rate on tool steel microstructure wherein the slower quench allows re-precipitation of carbide at the grain boundaries. This results in a loss of hardness and toughness of the material. One of the simplest and most effective ways to enhance the performance of PM and HSS grades is to utilize high pressure quenching (4 bar or greater).

Current Trends — Salt Bath Heat Treatment

Typical salt bath heat treat operation
Figure 3: Typical salt bath heat treat operation using high temperature neutral salts. Part is being removed from the high heat at 2175°F and will be quenched in the near pot operating at 1000°F.

Once the predominate method of hardening HSS cutting tools, high temperature salt baths serve a diminished but important role in the heat treatment of tooling materials. This is a basic method utilizing a refractory lined pot of molten salt as the heating medium. The salt is maintained in a neutral condition which protects the surface of the parts from significant oxidation and decarb. The furnaces operate constantly at a given temperature and a separate unit is required for each process step (first preheat, second preheat, high heat, quench, etc.). The parts being treated are manually moved from step to step. Quenching is accomplished by transferring the part from the high heat austenitizing temperature to a quench bath typically operating in the range of 950° to 1050°F from which point the parts can be cooled in air. This is an ideal scenario wherein the cooling is very rapid on the high end (good for structure) and then relatively slowly on the low end to minimize distortion.

Figure 3 shows an example of a salt bath hardening facility. A list of the pros and cons pertaining to this method might include the following:

Advantages of salt bath heat treatment

  1. Excellent means of heating and cooling due to liquid/solid heat transfer.
  2. Provides high heat temperature control and ideal quench.
  3. Good distortion control due to individual fixturing, minimal time at temperature, and step quenching.
  4. A solution for long thin tools such as broaches and feed screws which can be hung vertically and hot straightened out of the quench (before full transformation to martensite).
  5. Ideal for special or unique temperature and time requirements due minimal batching concerns.

Disadvantages of salt bath heat treatment

  1. Environmental issues with the barium salts.
  2. Quality and consistency are dependent on operator skill due to manual nature of the process
  3. Large amount of material handling required
  4. Some degradation of surface quality (oxidation and roughening).

The majority of the tool steel grades are processed by either of the methods outlined above. The generic atmosphere heat treating typically done for basic alloy steel is not generally employed in the case of the higher alloy tool steels due to temperature limitations. This type of furnace equipment is usually limited to a maximum temperature of 1900°F. Fluid bed furnaces using nitrogen as a process gas have slightly better temperature range, but these tend to be used more for surface treating than neutral hardening.

The most basic methodology for hardening tool steels entails the use of small electrically heated bench type “tool room” furnaces. The advantage here is low cost and quick turnaround. This equipment tends to be most suitable for lower hardening temperatures, but can be used for the higher end grades if care is taken. In either case, this type of processing usually involves small quantities of relatively small parts with the results obtained being dependent upon the care of the operator. It is essential that the equipment used be accurately calibrated and its heating characteristics be fully understood. Parts are usually wrapped in stainless foil to provide some amount of surface protection, and extra grind stock is usually necessary to insure a clean surface on the finished tooling.

In the next installment we will provide practical advice for maintaining control of heat treat quality. Some specific commentary will be included about how to select and work effectively with a commercial heat treat source.

We hope you find this heat treatment series informative. Should you have any questions or comments, please send them directly to gmaddock(at)zapp.com.

SERIES III: Basics of Heat Treatment • Part 6

Example of how salt bath remains an important method for certain tooling applications
Figure 1: Example of how salt bath remains an important method for certain tooling applications especially long parts, which require hot straightening.

Effectively managing the heat treat function requires a high level of involvement with the process. Simply calling out a Rockwell hardness and sending the parts out in the first truck to arrive at the dock is not generally a recipe for success. Granted that heat treat is an inconvenient, but vital process, that occurs in the middle of the often complex steps involved in the manufacture of tooling. It is amazing, from the tool steel supplier's perspective, how often the focus becomes logistics, turnaround time, and cost with little regard for the actual metallurgy necessary for a sound heat treatment process. While one might get away with this in the case of simple components and basic commodity tool steels, it is a sure fire recipe for downstream problems for tough applications involving highly alloy, high performance grades such as powder metals. A more involved, proactive approach will result not only in more consistent results, but can also differentiate the performance of your tooling relative to competitors who may be less concerned with the heat treatment process.

Figure 2: Example of how some delicate tooling components (Z M4PM) require special handling and creative fixturing to prevent warping and damage to sharp cutting edges

The first step to ensure quality heat treatment involves careful selection of a capable heat treat source. We would offer the following five questions to ask as one attempts to qualify a vendor to provide this service.

  1. What is their primary business focus? Granted many heat treat shops tend to be a jack of all trades, but one should be aware of the philosophical differences between production heat treat (high volume, repetitive parts, nuts and bolts heat teat) as compared to the care and attention required for tooling components. It is important to determine the dedication that a heat treater has to the heat treatment of tool steels. If possible consideration needs to be given to heat treaters who specialize at running tool steel and who maintain a broad array of capabilities developed specifically for this type of work. Is heat treatment of tool steels one of their core capabilities?
  2. Have they made investment in new equipment dedicated to tooling work? State of the art vacuum furnace technology represents a huge improvement (especially in regard to quench performance) over older equipment. Although heat treated parts may look the same after being run, those done in the more recent equipment will have much better metallurgical structure.
  3. Do they have capability for electronic data collection and process documentation? Control technology is another area where significant gains have been made in recent years, and information of this type can be hugely beneficial when attempting to deal with tooling issues and make improvements in tool life.
  4. Do they have a basic met lab? Running a heat teat without lab capability is like driving a car without a speedometer! Zapp's tech series has highlighted the matter of metallurgical structure, and a basic lab with metallographic capability is the means by which this critical aspect can be controlled.
  5. How do the intangibles compare? There are many factors to consider in qualifying a heat treat source including some that can provide a basic indication as to how they run their business. This would include housekeeping, lighting, organization, fixturing capability, etc. While not rocket science by any stretch of the imagination, the heat treatment of tooling is less forgiving and often does require extra care and attention along the way.
Example of how salt bath can be extremely effective method for niche grades and large parts that would be difficult to batch
Figure 3: Another example of how salt bath can be extremely effective method for niche grades and large parts that would be difficult to batch.

Given the appropriate choice of a well-qualified heat treat vendor, the following list of five suggestions can be offered as steps toward maintaining close control of quality as parts go through the process.

  1. Work with the heat treater (and steel supplier) to develop a written specification for the common materials being run. Specifications do NOT have to be so stringent that it backs the heat treater into the corner. It is always possible to design specifications such that parts can be effectively batched and still obtain desired results from a metallurgical stand point.
  2. Require certification for each job run. This sends a strong message that you are keeping an eye on the process. Measuring the process always improves the process.
  3. Inspect work when received from heat treat. This should include a quick hardness check as well as a visual examination looking for anything out of the ordinary. It is far better to identify concerns early as opposed later after incurring more costs in downstream operations.
  4. Periodically visit and audit the heat treat shop. This can be more for the purpose of maintaining an open dialogue and communication, but it is smart to ensure procedures are being followed as specified.
  5. Periodically run a sample through a lab to verify metallurgical structure. This entails having a sample sectioned, mounted and polished. This is the only way to absolutely judge the quality of heat treat and is certainly worth the effort in the case of critical components which get run through heat treat on a regular basis.
Digital control which alloys process display and electronic data collection
Figure 4: State of the art digital control which alloys process display and electronic data collection.

Final comments are in fact an impassioned defense for tool steel heat treaters who take this work seriously. The first would be to allow them the time to do perform the process correctly. A typical hardening cycle in a vacuum furnace can take 4 to 6 hours, and a triple temper cycle with a thorough cool down between each can take upwards of 12 hours or more. If your expectation is the heat treater should pick up your parts up in the afternoon and return them first thing the following day, you are indicating it is acceptable for some corners to be cut in the process. Proper heat treatment takes time. Along similar lines the matter of cost needs careful consideration. Heat treat expense in most cases is a relatively small percentage of the overall cost of tooling manufacture and given the importance for tool performance, it is not an area to cut corners in order to shave a few cents off the bill. In some cases, heat treat services are where one gets what they pay for. Finally, heat treaters who have invested in new equipment and technology need a fair return on their investment and deserve the support of customers who value good, quality heat treatment.

Hopefully this has provided some food for thought for toolmakers working with outside heat treat sources. Quality heat treatment is a valuable asset for your business.

We hope you find this heat treatment series informative. Should you have any questions or comments, please send them directly to gmaddock(at)zapp.com.