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Hardening of 80CrV2 in Bladesmith Forge
2022 Volume 62 Issue 11 Pages 2397-2401
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2022 Volume 62 Issue 11 Pages 2397-2401
Some bladesmiths have started to use 80CrV2 because V alloying helps with grain size. Smiths usually forge blades from unalloyed high carbon steels, but if Al deoxidation is not used, they are prone to grain growth. This work studies the forge hardening of 80CrV2 and C75 in the absence of Al. Specimens were heated in a gas-fired forge and austenitization was detected with a magnet. Both steel grades hardened when quenched after the disappearance of magnetism, but 80CrV2, due to Cr alloying, needed about 60°C higher temperature to attain maximum hardness. Grain growth did not start in 80CrV2 despite some overheating, while in C75 it started immediately after austenitization. The tests show that 80CrV2 attains easily good mechanical properties in forge hardening.
At the beginning of the 2000s, at Laurin Metalli knife blade factory, we replaced C75 with 80CrV2. Grain growth was an issue in C75, and the V alloying of 80CrV2 solved the problem. Laurin Metalli is the dominant player in the Finnish knife blade and knife steel market, so domestic bladesmiths adopted 80CrV2, and now its popularity increases abroad. Online material shops sell 80CrV2 for bladesmiths, and the smiths claim to get good hardening results with it. However, it does not have science-based forge hardening advice, and the differences between it and unalloyed blade steels are not fully understood.
Deoxidation Al can prevent grain growth in unalloyed carbon steels, and Al can be as effective as V,1) but the common standards do not define precisely the required Al content. For instance, C75S (C75) steel strips are made according to European standard EN 10132.2) The standard says “fine grain steel shall have an austenite grain size of 5 or finer” and it assumes that this requirement is achieved “when the total aluminum content is a minimum of 0.018%”. But there are not mandatory Al or grain size requirements. If the purchaser does not ask for a small grain size, then “the grain size shall be left to the discretion of the manufacturer”. This is not useful for blade factories and bladesmiths who use standard stock steels. Thus, V alloyed 80CrV2 is a more reliable selection.
This study is conducted from the viewpoint of puukko blades and similar. Puukko is a traditional Finnish wood carving and general purpose knife. The blade is often edge hardened (hard edge and soft back), which eliminates the danger of breaking the blade in two. The puukko blade generally dulls, according to my observations, because of microchips and dents, but not abrasive wear. Thus, the main concern with puukko steel is high hardness and good toughness. In this respect, unalloyed high carbon steels or low alloy steels, like 80CrV2, are the most interesting.
This study aims to make forge hardening advice for 80CrV2. The influence of the Cr and V alloying on hardening parameters and final properties are determined by comparing 80CrV2 and C75.
Steel materials were analyzed using an emission spectrometer, and the measured chemical compositions are presented in Table 1.
| C | Si | Mn | P | S | Cr | V | Al | |
|---|---|---|---|---|---|---|---|---|
| 80CrV2 | 0.82 | 0.26 | 0.42 | 0.007 | 0.003 | 0.60 | 0.195 | 0.005 |
| C75 | 0.74 | 0.24 | 0.63 | 0.010 | 0.010 | 0.03 | 0.000 | 0.002 |
C75 material was cut longitudinally from steel bar and specimens were hand forged. 80CrV2 was taken from a 4.2 mm thick steel plate. The thickness did not allow forging, but the material was heated to a forging temperature of 1100–1200°C (solution treatment). During the heating, Cr and V carbides dissolved and the industrial thermal history was erased.
All steel materials were normalized thrice. The three normalizing treatments were used for sure because 80CrV2 had an exceptional large grain size after solution treatment. Pieces (4.2 × 7 × 40 mm) were cooled on brick. The heat conductivity of brick is slower than that of steel. Therefore, the small pieces cooled down on a brick approximately as fast as a blade cools down on a steel anvil.
For 80CrV2, two different starting microstructures were additionally produced: tempered martensite and slightly coarser but still fine pearlite. The latter was obtained by normalizing a larger piece of plate (4.2 × 50 × 40 mm) and cooling it in still air before cutting the smaller specimens. The plate was bulkier than a typical puukko knife blade, so the cooling rate was slightly slower than that of an air-cooled puukko blade.
Before hardening, the deteriorated surface was removed and specimens were ground close to the final dimensions using an angle grinder and belt grinder. A hole was drilled at the end of the specimen, and the specimen was hung on an austenitic steel wire so that the heating and magnetic measurement were accurate.
The specimens were heated in a forge by gas torch flame. Two different hardening temperatures were used: non-magnetic and shade brighter heat color than non-magnetic. The former was executed by heating the specimen and intermittently testing whether it stops sticking to a magnet. When the non-magnetic state was detected, the specimen was brought back to the torch flame for one second to compensate for the temperature loss during magnetism measurement and then quenched. The latter hardening temperature was obtained by extending heating after the disappearance of magnetism by calculating four seconds and inspecting a slight brightening in heat color.
Specimens were quenched into hot (about 70°C) canola oil, then they were cooled on an anvil to room temperature, and after that immediately moved to a tempering furnace. Also, water quenching was tried but rejected because quench cracks were noticed. Tempering 1 h was made in a kitchen oven. The temperature was selected between 150 and 300°C and it was measured with a thermocouple. The heat treatment variables are listed in Table 2.
| Steel | Forging/heating | Pre-heat treatment | Forge hardening temperature | Tempering range |
|---|---|---|---|---|
| 80CrV2 | Solution heating | Normalizing in air Normalizing on brick Quenching in oil | Non-magnetic + extra | 150–300°C |
| C75 | Forging | Normalizing on brick | Non-magnetic Non-magnetic + extra | 150–300°C |
After heat treatments, the decarburized surface layer was removed by using a wet stone wheel and the final dimensions were achieved. The specimen thickness and width were 3.8 mm and 6 mm, respectively. The length was 35 mm or longer. Sharp edges were rounded because they may promote fractures. A fracture generally started at the center of the front surface.
2.3. MeasurementsIn the bending test, the specimen end was clamped in a vise and the specimen was bent with a wrench until it broke into two. In the case of large bending angles, sufficient bending was not achieved using the wrench only because of the thickness of its jaws. Then, the final breakage was executed by using a hammer blow. Two pieces of the broken specimen were refitted together and the ductile bending (angle in the fractured specimen) was measured. An example of the angle measurement is presented in Fig. 1. The used test method has been introduced earlier.1)
The two halves of the broken bending test specimen are refitted together so that the bend angle can be measured.
Grain size was rated by the fracture grain size method, developed by Arpi and Shepherd.3,4) The fracture surface of as-quenched or low temperature tempered high carbon steel specimen reveals prior austenite grain size. Fractures with ASTM grain size numbers from 1 to 6 are fully intergranular and from 7 to 10 the portion of intergranular fracture decreases to zero.4) Grain sizes finer than 10 can not be rated because they do not produce feature differences discernible by eye.3,4)
Hardness was measured by Rockwell C test.
When C75 specimens were quenched immediately after the disappearance of magnetism they attained 65 HRC hardness and the fracture grain size was the finest possible (ASTM 10). When specimens were additionally heated about 4 s after the loss of magnetism, the as-quenched hardness was not higher but the fracture surface was slightly coarser (ASTM 8). Thus, the optimal hardening temperature for C75 seems just non-magnetic.
80CrV2 steel plate was received from the factory in a soft annealed condition. The soft annealed initial microstructure is not good for forge hardening because large spheroidized carbides need too long soaking time. But anyway, it was tested, and as expected, the reaction to heating was slow and hardening was difficult to execute. Further hardening tests were made with normalized initial microstructure.
The hardness of 80CrV2 depends on the austenitizing temperature because Cr alloying makes it slightly hypereutectoid. When quenched from non-magnetic temperature, it reached only 64 HRC hardness. The maximum as-quenched hardness of 66 HRC was attained when a specimen was additionally heated 4 s or more after the loss of magnetism.
The fracture grain size of 80CrV2 remained superfine when additional heating was about 4–6 s, and an increase in grain size was observed when the heating was 8 s. When 16 s was used, individual grains were visible. Examples of as-quenched fracture surfaces are shown in Fig. 2.
Examples of fracture surfaces of as-quenched 80CrV2 specimens when different heating times after the disappearance of magnetism were used. The specimen, above, middle, and below had 16, 8, and 4 s additional heating, respectively.
The 4 s additional heating with torch flame was optimal in this case. It produced a small but noticeable increase in the brightness of heat color. Smiths universally use magnet and color temperatures. Thus, the universal advice for the hardening temperature for 80CrV2 is the disappearance of magnetism plus a shade brighter heat color.
The hardening temperature was also measured with a thermocouple whose end was drilled into the specimen. When the specimen stopped sticking to a magnet, the thermocouple showed temperatures of 770–790°C. After 4 s of additional heating, it showed 820–860°C. Thus, the shade brighter heat color was about 60°C.
Figure 3 shows the hardness as a function of tempering temperature. The steels were quenched from the optimal hardening temperatures and they had maximum as-quenched hardness before tempering.
The hardness of 80CrV2 and C75 as a function of tempering temperature. (Online version in color.)
The microstructure of C75 specimens before hardening was always brick-cooled pearlite. Two different austenitizing temperatures were used: non-magnetic and about 60°C higher than that. These austenitizing temperatures produced fracture grain sizes ASTM 10 and 8, respectively. The former had better ductility, as shown in Fig. 4, but the relative difference decreased with decreasing hardness and at the level of 56–55 HRC (tempering 270–300°C) the difference became unimportant in the respect of knife blades.
The bending test results for hardened and tempered C75 specimens show how the increase in grain size from ASTM 10 to ASTM 8 decreases ductility. The effect is detrimental at high hardness levels, but it becomes practically unimportant at low hardness levels. (Online version in color.)
For 80CrV2 specimens, three different initial microstructures were used: normalized with fast cooling on a brick, normalized with slow cooling in still air, and hardened. All specimens were austenitized at about 60°C higher than non-magnetic temperature to attain maximum as-quenched hardness. The used initial microstructures did not affect the fracture surface of hardened specimens, which was always superfine, nor did it affect ductility, as shown in Fig. 5.
Bending test results for hardened and tempered 80CrV2 specimens with different initial microstructures. Fine pearlite (cooling in still air), very fine pearlite (cooling on brick), and martensite resulted in similar ductility. The fracture grain size was always ASTM 10. (Online version in color.)
A concern in determining austenitization with a magnet is that the ferrite phase loses its ferromagnetic properties at Curie point at around 770°C. The Curie point may overlap the ferrite to austenite transformation.
When a heating rate is extremely slow, austenitization occurs at about 730°C (Ac1), but with practical heating rates, it takes place at higher temperatures. In the current study, a temperature meter showed 770–790°C when the loss of magnetism was noticed. Specimens did harden from non-magnetic, so it seems that the ferrite phase retained some ferromagnetism until the completion of austenite formation. The optimal forge hardening temperature for 80CrV2 was found to be 820–860°C, which is clearly above the Curie point.
4.2. Carbon SteelsIn my earlier work grain growth was not avoided when completely unalloyed carbon steel was hardened in a gas forge.1) Then tongs were used, but now a specimen was hung on a steel wire. With the current technique, the instant quenching from non-magnetic was more accurate and the grain size remained superfine despite the absence of Al.
When about 60°C higher than non-magnetic hardening temperature was used, C75 specimens got a similar fracture grain size (ASTM 8) to the unalloyed carbon steel in my earlier work. It also behaved similarly. After tempering at low temperature it was very brittle, but when tempering temperature was increased, it reached the ductility of steel with superfine grain size.1)
As-quenched or low temperature tempered high carbon steel tends to fracture intergranular manner because of the formation of cementite on austenite grain boundaries during quenching. This phenomenon is called quench embrittlement.5,6) It does not occur if the grain size is small enough.4,5,6) The steel with a grain size of ASTM 10 avoided quench embrittlement, and the steel with ASTM 8 suffered it. However, when C75 was tempered at 270–300°C the quench embrittlement vanished and both grain sizes produced good ductility (Fig. 4).
The quench embrittlement is a different phenomenon than tempered martensite embrittlement (TME), which is a result of the formation of cementite during tempering at around 250°C. With carbon steels, TME is regularly noticed in torsion impact tests but not in the unnotched Charpy test.7) The latter is more relevant for bladesmithing. TME is not an issue in traditional bladesmithing, and carbon steel blades are tempered in the TME range. My previous1) and current bending tests did not reveal TME.
4.3. 80CrV2The austenite grain size depends on the pre-heat treatment. Faster cooling on normalizing produces smaller pearlite grains, which in turn produces smaller austenite grains. Presumably, martensitic starting microstructure produces the smallest austenite grains. Grain size ASTM 12 can be expected in double hardened specimens.8)
However, the used pre-heat treatments did not affect the ductility or the appearance of the fracture surface. This can be explained as follows. With grain sizes about ASTM 10 and smaller, fracture mode is fully transgranular.4) Because fracture does not propagate along prior austenite grain boundaries, the effect of grain size is insignificant. Unfortunately, the metallographic measurement of actual austenite grain size was not available in this study.
Whether the normalized blades should be rapidly cooled on an anvil or slowly cooled in still air is not well established. The current study demonstrated that 80CrV2 blades can equally be cooled on an anvil or in still air. The fact that Cr alloying delays pearlite formation may help to produce small pearlite grains when 80CrV2 is slowly cooled in still air.
80CrV2 hardened when quenched from non-magnetic temperature 770–790°C and full hardness was attained when quenched from 820–860°C. These values are quite similar to those recommended for industrial furnace hardening. EN 10132 standard advises hardening temperatures of 840–870°C for 80CrV2.2) An old but famous ASM Tool Steels handbook advises hardening temperatures of 760–840°C for L2 steel whose chemical composition (0.8C-0.25Mn-0.8Cr-0.2V) is nearly similar to 80CrV2. According to the handbook, the temperatures at the lower end of the range are for water quenching, and they do not dissolve a sufficient amount of carbon for full hardness.9)
Steel attains maximum as-quenched hardness when martensite contain 0.8–0.9% carbon. If the carbon content increases beyond this, the amount of retained austenite increases to an excessive level (>10%) and hardness drops.10) Alloying elements also increase retained austenite, so alloyed oil-hardening steels have lower as-quenched hardness than unalloyed water-hardening steels. 80CrV2 attained 66 HRC, which is about the absolute maximum for oil-hardening steel, while water-hardening steels can reach about 67 HRC.
Alloying with Cr may promote the occurrence of TME so that it is more likely in 80CrV2 than in unalloyed carbon steels. A lower carbon variety L2 (50CrV4) had TME in torsion and unnotched Charpy test, but it was nearly absent in notched Izod test.11) In 5160 steel (60Cr3) TME was present when tested with Charpy V and studied metallographically.6) Using unnotched Charpy tests, I found TME in 80CrV2 but not in unalloyed carbon steels.12) The bending tests in the current study did not reveal TME in 80CrV2.
In addition, 80CrV2 blades can be tempered below the TME range, particularly the edge hardened ones which have an unbreakable spine. Due to superfine grain size, 80CrV2 edge attains sufficient ductility after a low temperature tempering. According to my experience in the field, tempering at 170°C is sufficient for normal wood carving, and for more severe work tempering at 200°C is better. These are lower than the temperatures (200–300°C) generally used by bladesmiths. Thus, an 80CrV2 blade can be harder and in that way better than a typical puukko knife blade.
Some bladesmiths have started to use 80CrV2 instead of unalloyed steels. The current work compares 80CrV2 with C75 and reveals forge hardening advice for 80CrV2. The studied C75 had no Al deoxidation. Despite that, careful temperature control prevented grain growth. However, it was difficult and not necessarily repeatable in real-life bladesmithing. A smith who uses unalloyed carbon steel without Al has a high risk for grain growth and unsuccessful hardening results. Forge hardening of 80CrV2 is easy because V prevents grain growth, so bladesmiths can repeatedly get good hardening results with it. The current work emphasizes the following five points for forge hardening of 80CrV2 steel:
(1) Commercial steel plates are often soft annealed. It is not a suitable starting microstructure for forge hardening. The material shall be forged (or solution treated) and normalized before hardening.
(2) Normalizing with cooling on anvil or in air.
(3) Cr alloying increases the required austenitization temperature. The optimal forge hardening temperature is 820–860°C, which is one shade brighter heat color than non-magnetic.
(4) Oil quenching (water quenching may result in cracks).
(5) Low tempering temperatures of 170–200°C can be used because of superfine grain size and good ductility.
