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Phone: 1 888 928 9927
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TOOL STEEL
TECHNICAL LIBRARY

SERIES I: Introducing the Concept of Tool Steel Microstructure

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

SERIES III: Basics of Heat Treatment • Part 1

SERIES III: Basics of Heat Treatment • Part 2

SERIES III: Basics of Heat Treatment • Part 3

SERIES III: Basics of Heat Treatment • Part 4

SERIES III: Basics of Heat Treatment • Part 5

SERIES III: Basics of Heat Treatment • Part 6
 

ZAPP HIGH-PERFORMANCE
STEEL GRADES:

TOOL STEELS
Z-TUFF PM
Z-WEAR PM
Z-A11 PM
Z-A11LV PM
Z-420 PM

HIGH SPEED STEELS
Z-M4 PM
Z-T15 PM
Z-M48 PM
Z-MAX PM
Z-M2
 

ZAPP PERFORMANCE PRODUCTS:
ZDM BLANKS
 

TOOL STEEL TECHNICAL TRAINING

SERIES III: Basics of Heat Treatment • Part 5

Current Trends — Vacuum Processing

Figure 1

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.

Figure 2

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

Figure 3

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@zapp.com.

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