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Video
I also have a video covering this same material:
Cold Forging and Rolling
I already have a separate article about the mechanisms behind cold forging and rolling, so I will not recap much of that here: Cold Forging of Steel.
While it is called “cold” forging it doesn’t usually refer to freezing or subzero temperatures. Most commonly, cold forging refers to room temperature forging. There are several major differences with high temperature forging; here are a few important ones:
- The steel requires higher forces to forge.
- The steel increases in hardness through forging and eventually will fracture from too much deformation.
- The surface of the steel remains untarnished – no scale or decarburization.
Cold forging is not particularly rare or exotic. In fact some knife steels are available in the cold rolled condition. However, some knifemakers will cold forge their blades, at least partially. This is a somewhat less common practice. Cold forging works the same way in practice as cold rolling though of course cold rolling leads to a more even distribution of the cold reduction to the steel.
Some knifemakers have claimed performance benefits from the cold forging. In the previous article I wrote, I cited previous studies performed with high carbon steels showing some small potential benefits to heat treating steel from a cold forged/rolled condition. These improvements are generally explained by a reduction in grain size by performing the cold reduction before the quench and temper procedure. However, I had not done any experiments to see the improvements compared with other processing changes we have tried. Now we have done a study to see it for ourselves.
The New 52100 Study
We used Uddeholm 52100, close to the mid-point composition. There were two conditions tested:
- As-received – The steel as it arrives from Uddeholm, annealed and ready to use.
- Normalized at 1700°F (925°C) for 20 minutes and air cooled. Grain refinement treatment from 1460°F (795°C) for 30 minutes, air cool. Annealed at 1460°F for 30 minutes, cooled at 650°F/hr (350°C/hr). We call this anneal a “Fast DET” anneal where DET stands for “Divorced Eutectoid Transformation.” It is “fast” because the cooling rate is much faster than generally recommended in datasheets. You can read more about normalizing and annealing in this article.
Each of those two conditions were then cold rolled either 20% or 40% and compared to zero cold reduction. Cold rolling is usually best performed from an annealed condition so that it is as soft as possible to avoid fracturing during cold reduction. Perhaps you can get away with a normalized condition depending on the steel and how much cold reduction will be performed. Steels that have large carbides and relatively low toughness in the annealed condition are not good choices for cold forging.
We then performed a final quench and temper heat treatment with 1500°F (815°C) for 15 minutes, quenched in Parks 50 oil, and tempered twice at 400°F (200°C) for one hour each time.
Hardness After Cold Rolling
The annealed hardness is somewhat higher with the fast anneal. However the finer microstructure from this anneal leads to a better hardness-toughness balance after the final heat treatment as you will see. However, somewhat surprisingly the hardness after cold reduction was very close for the two different prior conditions. The increase in hardness was roughly similar to a study I cited in the cold forging article with A8 Mod tool steel [1]:
Data adapted from [1]
Hardness After Final Heat Treatment
After the quench and temper, you can see that the cold reduction led to higher hardness. The difference was relatively small, however, even with 40% cold reduction, 0.7 Rc for the as-received and 0.3 Rc for the normalized and annealed. Cold reduction leads to faster dissolution of carbides putting more carbon in solution and therefore higher hardness after heat treating. In this case the finer microstructure achieved through our modified annealing procedure led to a bigger difference in hardness than cold rolling.
Toughness
The cold reduction led to lower impact toughness for either the as-received or thermal cycled conditions. This is perhaps expected based on the increase in hardness though we had hoped that there would be a reduction in grain size to help improve toughness despite the small increase in hardness. The two different conditions, as-received or thermal cycled, had similar toughness despite the thermal cycled condition leading to higher heat treated hardness.
Hardness-Toughness Balance
The thermal cycled condition (Normalized, Fast DET anneal) had ~1 Rc higher hardness but similar toughness to the as-received condition. Thus the hardness-toughness balance was significantly better in the thermal cycled condition. However, the cold reduction did not improve hardness-toughness to the same extent as an improved initial carbide structure did.
The effects of cold reduction look more similar to a typical reduction in toughness (with higher hardness) from a lower tempering temperature or adding a cryo step as shown below. You can see where the following numbers came from in the earlier article about heat treating 52100.
Why No Improvement?
Because there are a few former studies that showed a small improvement in toughness from cold reduction before heat treating I wouldn’t totally discount it. However, positive results are more likely to be published and I did not find an improvement in this testing. Looking at those studies the measured improvements were quite small and perhaps were just within the scatter of the test.
Maybe in this case the increase in carbon in solution was detrimental enough to overwhelm any other improvement we might have obtained. In the previous Cold Forging article I wrote there was a case with D2 where they found a reduction in hardness from cold reduction because extra carbon in solution meant more retained austenite. So as with any change to processing there can sometimes be unintended consequences. Very rarely are there changes that only improve things without any potential detrimental effects. Perhaps if I slightly reduced the austenitizing temperature of the cold rolled material that would lead to similar hardness and thus either similar toughness or slightly improved toughness.
Summary and Conclusions
Cold reduction led to an increase in hardness of the annealed 52100 as well as the quench and tempered 52100. However, the increase in hardness led to a small reduction in toughness. There was no clear improvement in the hardness-toughness balance. The increase in hardness comes from higher carbon in solution, so perhaps a small reduction in the austenitizing temperature would help to compensate. The thermal cycled 52100, with or without cold reduction, had superior hardness-toughness balanced to the as-received steel. The fast annealing procedure utilized in its processing led to a finer carbide structure which improved its properties.
[1] Ghasemi-Nanesa, Hadi, Mohammad Jahazi, Majid Heidari, and Tom Levasseur. “The influence of deformation-induced microvoids on mechanical failure of AISI A8-Mod martensitic tool steel.” In AIP Conference Proceedings, vol. 1896, no. 1, p. 020021. AIP Publishing, 2017.
