Hand Tools Archive

Subject:
Lamination and quenching in water-hardening steel

Steve Voigt
I stayed out of the A2 discussion because frankly I'm not interested in rehashing that topic any more.As much as I disliked Charlesworth's post, I just don't care any more. But the following quote from Wiley really caught my eye:

The metallurgist John Verheuven, in Steel Metallurgy for Non-Metallurgists, shows quenching data that demonstrates the problem in through-hardening unalloyed high carbon, such as 1095 (same as W1), when the thickness gets above 0.09” (3/32”). When you get up toward 1/8” or 0.145”, it takes extremely fast quench times to avoid the formation of pearlite or bainite in preference to martensite. And the problem gets worse with finer-grain steel.

The common explanation for why Japanese irons/chisels/etc perform so well is that the unhardened backer (mild steel or wrought iron) acts as sort of a shock absorber, allowing the cutting edge to be left very hard (around 63 RC) without suffering from chipping. For what it's worth, I have never bought this explanation.It's like claiming that great shock absorbers allow you to drive over nails without getting a flat tire.

An alternative explanation is that the greater carbon content of Japanese steel (roughly 1.2% vs 1% in W1 or 1095) is the difference-maker. I'm skeptical of this as well. The japanese tools are much more intensively forged than western tools usually are, so I believe much of the excess carbon is burnt off during forging. And if the steel is till hypereutectic (i.e. above roughly .75% carbon) when it finally gets heat-treated, the excess carbon is not going to get used unless the steel is kept at the critical temperature (roughly 1450 F) for at least a few minutes--something that is not happening at the one man shop level, where the smith is heating the steel up to critical temp and immediately quenching it.

So, we are left with the explanation Wiley (via Verheuven) is suggesting: that lamination keeps the cross section of tool steel very thin, and as a result it gets hardened more completely than would be possible with a solid chunk. This is the only explanation that has ever made sense to me.

I will tentatively suggest, based on my own limited experience, that laminating the tool steel to a big chunk of wrought iron also constrains the fromer from warping and distorting during HT, something that is hard to avoid with a solid chunk of O1.

I guess I don't really have a point here, but I'd love to hear more thoughts from Wiley or anybody else.

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