How to heat treat Damasteel® RWL34

RWL34 is a Rapid Solidified Powder (RSP) martensitic stainless steel, hardenable and non-patterned steel. The alloy represents an excellent combination of corrosion resistance and hardenability.

RWL34 is named after the world-famous knife maker Robert (Bob) W. Loveless. Bob Loveless enjoyed using the Japanese ATS-34 steel. Working with Pelle Billgren, founder of Damasteel, ATS-34 was improved using their unique powder metallurgy, creating an exceptionally high quality and high purity steel named RWL34.

This steel is also used in Damasteel’s DS93X stainless damascus together with PMC27 (Read my heat treating guide here).

RWL34 Chemical Composition (nominal) %:

RWL34 is a variation of the 420 martensitic stainless family that contains 14% chromium, molybdenum and vanadium for improved corrosion resistance, hardness, and strength. The performance of these alloys is further enhanced by the RSP process, which involves using high-pressure nitrogen gas to force the molten steel through a nozzle.

This advanced alloy mix makes it a very close equivalent of 154 CM and ATS-34 stainless steels, but it has the additional advantages offered by the RSP process (powder metallurgy), it is closest to CPM 154, Crucible’s particle metallurgy version of 154 CM with the addition of several trace elements.

Here is a composition comparison graph of the steels usually compared to 154CM, ATS34 (visit zKnives.com to view a side-by-side graph).

Heat Treatment Overview

NB! It’s important to protect the steel from oxidation and decarburization during hardening. Cordusal, Turco, and ATP-641, (anti-scale compounds) and stainless steel tool wrap, are probably the best choice unless you want to spend time removing the affected surface scale post-heat treatment.

Thermal cycling / Annealing (optional):

• An often overlooked step, but due to its susceptibility to warping during oil hardening, an annealing step can save you countless headaches.

1. Normalizing: Heat the blade (780-840°C/1600°F) in the furnace and hold for 20 minutes. Allow the blade to air cool to room temp. The resultant microstructure ideally will be pearlite [1].
2. Annealing (Ferritic): 770°C (1420°F) for 3 hours soak and cool very slowly in the furnace. Hardness will be about 300 HV. A Protective atmosphere or foil/tool wrap can be used to avoid decarburization.
3. Annealing (DET) Recommended:
   • 910°C (1670°F) for 2 hours. Slow cooling by 15 deg C/hour (59 deg F/hour) down to 750°C (1380°F), and hold for a further 2 hours before cooling to room temperature (air). Hardness will be below 250 HV. A Protective atmosphere or foil/tool wrap should be used to avoid decarburization and scale.
   • Old Version: Transformation annealing starts at 865°C (1410°F) and after that a slow cooling by 10 deg C/hour (20 deg F/hour) down to 700°C (1300°F).

Many studies that used far higher temperatures (>1100°C/2012°F) for the austenitising temperatures. While this reportedly resulted in the hardest / highest HRC values many of those studies did also point out that it also resulted in large grain sizes. The large grain size reduces toughness and for knife proposes, is something we generally try to avoid.

Austenitizing / Hardening

• Pre-heating: 780°C (1440°F) – 10 minutes (large or complex parts)
• Hardening Temp (Furnace):
  1. No Cold Treatment Used/Available: 1050°C/1925°F
  2. Cold Treatment (see cryogenic section): 1080°C – 1100°C (1975°F – 2010°F).
• Soak time: 15min (3mm) up to 60 minutes (25mm) Max

The danger of an insufficient soak is much worse, leading to poor hardness, toughness, and corrosion resistance. While the chances of an overly long soak time are quite low. The changes to the steel in terms of transformation and carbides dissolving will “level off” after a certain amount of time and then changes are very slow after that. So it is recommended that the soak is “long enough” rather than trying to make it as short as possible.

Quenching media

• Recommended: A medium-speed oil (warm to 125°F / 50°C) can be used until the part is black.
• Aluminium plates and compressed air to or below 125°F / 50°C (cooling below 300°C in 2 minutes or faster)

Cryogenic Processing

Cryogenic treatment or Deep cooling (-80°C/-112°C) is recommended to convert retained austenite, which should be done before the first temper cycle. Hold for at least 2 (two) hours.

While liquid nitrogen is preferred, a sub-zero bath with dry ice and kerosene will suffice for -100°F / -74°C. Submerge in sub-zero treatment for 4 to 6 hours depending on thickness and number of blades. Clamping is recommended to avoid thermal shock-induced warp.

Any cryo treatment (whether between tempers, or after the quench, should always be followed by a tempering cycle.

True story

Tempering

Once the blade is quenched and near ambient temperature, blades should be tempered accordingly, the times suggested are to ensure an even, consistent temperature.

*If using a small toaster oven or household kitchen oven for tempering, using a blade holding rack made from kiln furniture, a roasting tray lined with fine sand, or a similar large object will help retain thermal mass to reduce wide swinging temperatures as the device fluctuates trying to maintain temperature.

• Tempering Temperature Range (Recommended range is between 150°C – 220°C (300°F – 430°F):
• Times: One (1) time
• Duration: Two (2) hours

If you’re using a high-temperature temper (520°C/970°F), which Damasteel recommend for maximum edge sharpness (whatever that is…), you’ll need to triple temper for 1 hour each time (3 x 1 hour). Generally for knifemaking, secondary hardening / high-temperature tempers aren’t recommended and can insight embrittlement, leading to a reduction of toughness and corrosion resistance (should not be used for food handling applications).

HRC Summary Table:

So there you have it, if you were looking to find out how to heat treat Damasteel® RWL34, hopefully, I’ve answered most if not all of your questions.

• 63.5 HRC: 1100°C/2010°F Austenitizing, Deep Freeze, 175°C/345°F Temper 1 x 2 hours
• 63 HRC: 1080°C/1980°F Austenitizing, Deep Freeze, 175°C/345°F Temper 1 x 2 hours
• 62 HRC: 1050°C/1920°F Austenitizing, 175°C/345°F Temper 1 x 2 hours
• 59 HRC: 1050°C/1920°F Austenitizing, 220°C/430°F Temper 1 x 2 hours

Disclaimer:

Please note that your heat-treating kiln will also differ in its readings compared to mine. As such it’s best to do your own testing (coupons). All the information above is from my readings of research papers, forum posts, and discussions with people. I am not a metallurgist and the above is presented here for the benefit of all knifemakers. You do not have to follow them and I’ll not be held responsible for any loss or damage you may experience.

Please comment below if you have anything to add to the above.

Links / References:

1. Damasteel Heat Treatment and Properties
2. RWL34 Datasheet (New)
3. High-Speed Steels Produced by Conventional Casting, Spray Forming and Powder Metallurgy