How to heat treat 5160 Spring steel

SAE 5160 (JIS G4801) is a very popular medium to high carbon (0.60%) chromium spring steel primarily due to its cost, toughness and resilience in truck leaf springs. As 5160 is very tough it is unsurprisingly used for swords (hardened in the low 50s Rc), large knives, including bowies, choppers and everything in between. In recent years more knifemakers are using 80CRV2 instead as it is arguably better suited (“5160 on steroids”) for knife blades.

I made my competition chopper out of 8mm 5160 and it’s held up to every test thrown at it.

The material is very flexible as it can be heat treated to various hardness to meet various operating environments. The chromium that is present in AISI 5160H is not at high enough levels to reduce its susceptibility to corrosion (<13%), therefore it is important to coat the knife blade with a rust preventative (Fluid Film) or a protective coating to ensure the material is not exposed directly to the elements.

SAE 5160 Chemical composition (nominal) %:

Chromium is the major alloying element in 5160 steel and also includes other key elements:

C 0.60, Mn 1.00, Cr 0.90, S 0.040, P 0.35, Si 0.30

Here is a composition comparison graph of the steels that are usually compared to 5160 (ie: 80CrV2, L2, 1084, O1) (visit zKnives.com).

Heat treatment

With a hardening capability of 57-63 HRC (840 HV), 5160 is very hard steel.

NB! It’s important to protect the steel from oxidation and decarburization during hardening. Cordusal, Turco, and ATP-641, (anti-scale compounds), are probably the best choice unless you don’t want to spend time removing the affected surface scale post-heat treat.

Thermal cycling (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 (870°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 [3].
2. Annealing: 677°C/1250°F for 2 hours
3. DET (Divorced Eutectoid Transformation): Heat the blade (750°C/1380°F) for 10 minutes, and allow the blade to cool in air. Unlike the above, we are looking for ferrite and carbide structures [3].

Austenitizing / Hardening

• Hardening Temp (Furnace): 829°C-850°C [1,525°F – 1562°F]
• Sweet spot: 840°C [1544°F]
• Soak time: 5min (2.5mm) to 10min (5mm) up to 30 minutes (12mm) Max

Quenching media

The cooling time necessary to suppress the bainite phase and obtain fully martensite must be extremely fast, increasing the possibility of the appearance of cracks, fractures, and the generation of residual stresses due to high-temperature gradients. [4]

Quenching in oil will give perfectly acceptable results with far less chance of blade failure when compared to water.

• a Fast-speed oil is recommended until the part is black (22% RA).
• Some sources [5] recommend interrupted water quench (1.5sec in heated brine (15% solution @ 100°C)) for thicker cross-sections – use a lower austenitizing temp (reduce the danger of cracking) with high agitation to reduce the film boiling stage.
• Others [6] show that Water + Soap (15%) produce a cooling rate that results in martensite and some retained austenite 14.9%.
• Aluminium plates or any clamping system can be used as a secondary quench step to minimize any warping.

The Isothermal Transformation Diagram below shows the time needed (how fast a quench) need to avoid forming ferrite, bainite and or cementite.

Cryogenic Processing

Despite the Mf point being below room temperature, one would expect cryogenic treatments to yield so positive effects (reduced RA, higher HRC, lower toughness) The results [8] showed that the cryogenic cycles used did not provide positive effects on spring steel performance. Without full access to this paper, the results are probably aimed at fatigue and impact resistance in springs rather than knife blades. If ultimate toughness is your goal, you should skip cryo.

Tempering

The objective of tempering is to reduce hardness and increase ductility while internal stresses are relieved. Furthermore, during the tempering process, an isothermal transformation of retained austenite to bainite usually takes place. In the case of the AISI 5160 steel the Mf is below room temperature, thus retained austenite cannot be avoided when quenching at room temperature.

Martensite embrittlement (TME) is the transformation of retained austenite to cementite among martensite laths and has been reported to occur between 250°C to 370°C and 450-600°C. [5]

• Tempering Temp:
  • 61HRC: 150°C (300°F)
  • 60HRC: 180°C (350°F))
• Times: 2 times
• Duration: One (1) hour each time

Recommended Hardness: 58 – 60HRC (after tempering) depending on the intended use. Lower HRC is recommended for tough prybars and higher hardness for sharp slicers that will see little chopping.

Disclaimer:

Please note that there are various manufacturers of 5160 whose tolerances/chemical composition will differ quite a bit, making exact heat-treatment specifications difficult to pinpoint for everyone. Your own heat treating kiln will also differ in its readings compared to mine. As such it’s best to do your own testing (coupons). All info 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. How to Heat Treat 5160 – Optimizing Toughness
2. Ranking Toughness of Forging Knife Steels
3. How to Thermal Cycle Knife Steel
4. Analysis of the effect of immersion rate on the distortion and residual stresses in quenched SAE5160 steel using FEM
5. EFFECT OF INTERRUPTED QUENCHING ON THE FATIGUE RESISTANCE OF AN AISI/SAE 5160 STEEL
6. The Effect of Varying Quenching Media on Cooling Time and Microstructures of Leaf Spring Steel AISI 5160
7. Bainite vs Martensite – The Secret to Ultimate Toughness?
8. Influence of Deep Cryogenic Treatment on the Mechanical Properties of Spring Steels
9. Effect of hardening and tempering on microstructure and mechanical properties of steel grade AISI 5160