The Doctor has always been curious about questions such as this one, which stimulate thought and promote an exchange of new ideas. Most, if not all, heat treaters – whether they like to admit it or not – enjoy the challenges that present themselves when confronted by a new material or the need to develop a new process recipe to achieve specified properties or results. Here, we will consider an example of doing just that. Let’s learn more.

A question was asked of The Doctor as to the best heat-treatment practice to develop Rockwell hardness in the low 60s for a Böhler N690 material. Being somewhat unfamiliar with this particular grade and what products are commonly manufactured from it, it was natural to do an Internet search to glean some general insights followed by a more detailed investigation of what heat-treat recommendations were available from the manufacturer. In addition, it made good sense to reach out to several commercial heat treaters to learn from their experience. As a result, several heat-treat recipes were developed.

Composition

Böhler N690 is classified as a stainless steel – more specifically, a martensitic chromium stainless steel with cobalt, molybdenum and vanadium additions (Table 1). The cobalt addition in particular is responsible for many of this alloy’s most desirable properties. This material is intended for applications requiring high hardness in combination with good corrosion and wear resistance, along with excellent surface-finish and edge-retention characteristics in the finely ground and highly polished condition.

Applications

Knife blades are one of the most common uses of this grade. Other applications include surgical (cutting) instruments, rotary knives for the meat-processing industry, plate and knife-edge fulcrums, corrosion-resistant roller bearings, valve needles and pistons for refrigeration systems.

Many knives are made of 440C steel, given that this alloy is easy to re-sharpen and has an excellent price point and good general-duty performance. However, the ability to hold a sharp edge in demanding field use and a desire to improve overall performance drives knife manufacturers to investigate alternative steels. As such, literally dozens of types of steel are used to create knife blades. Some of the more popular material grades include 420HC, 1095, AUS-8, 18Cr13MoV, X50CrMoV15 (DIN 1.4116), 154CM, S30V, VG10, D2, Elmax®, CPM3V, ZDP189 and SM100. Another popular choice is Böhler N690 (Fig. 1).

Most knife makers use VG10 to compare the performance of N690 rather than 440C. N690 belongs to a class of materials of similar chemistry (Table 1), which are all available in sheet form to facilitate manufacturing (i.e., stamping of the material).

Heat Treatment

Manufacturers typically provide general heat-treat instructions for their stainless steels with expected results in terms of microstructure and hardness (Table 2). Examples are:       

  • Annealing: 800-850°C (1470-1560°F). Resultant microstructure is ferrite and carbide.
  • Hardening (furnace): 1030-1080°C (1885-1920°F) followed by rapid forced-air cooling or oil quenching (for thick sections).
  • Tempering: 100-200°C (210-390°F). Resultant microstructure is martensite and carbide.

From this type of data, one can develop specific heat-treat recipes based on a given steel. For N690 knife blades this would be as follows:

  1. Preheat to 650-705˚C (1200-1300˚F) and equalize (i.e., soak until parts and load are uniform in temperature, typically 15-30 minutes).
  2. Heat to 815-870˚C (1500-1600˚F) and equalize (i.e., soak until parts and load are uniform in temperature, typically 10-20 minutes).
  3. Heat to 1060˚C (1940˚F) and soak for 15-30 minutes. Do not over soak; alternate hardening temperature: 1070˚C (1960˚F).
  4. Quench in air using rapid forced convection cooling.
  5. Deep freeze (for retained austenite conversion) at -84˚C (-120˚F) for a minimum of 2 hours with 3-4 hours recommended. The minimum treatment is -70˚C (-95˚F) with a soak of 3-4 hours at temperature; -185˚C (-300˚F) is also permitted; deep freeze within 30 minutes of quench completion.
  6. Temper immediately upon return to room temperature. Double tempering is required to achieve desired hardness and maximize stability. The minimum time at temperature is 2 hours.

Hardness results vary somewhat with the choice of austenitizing temperature and tempering temperature (Fig. 2). For example, to achieve 60-61 HRC, one would double temper N690 austenitized at 1060°C (1940°F) for two hours at a temperature of 99-135°C (210-275°F). By contrast, using an austenitizing temperature of 1070ºC (1960°F), one finds that a tempering temperature of 125-150°C (255-300°F) is required to achieve the same result.

Summary

What is important for the heat treater to focus on is the process of determining the best heat-treatment practice for a given steel. This involves understanding the end-use application, researching the material properties, and then creating an optimized recipe based on input from the steel supplier and the experience of the heat-treat community.

 

References

  1. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012
  2. Böhler N690 Technical Data Sheet DE 12.2007 EM-WS
  3. Wikipedia (wikipedia.org)

More about Knifemaking

Our website contains two interesting write-ups on knives.
The first can be found here: www.industrialheating.com/knives, and the second on Damascus steel knives here: www.industrialheating.com/damascus