Hardened cutting blade

[kid_108]

Just a quick input. Tempering is not a hardening process. Tempering is used to provide some ductility (softer) to something that has been hardened.

As far as using carburizing flames and oil quenches, that will not help. you are not at high enough temperatures and you don't have enough carbon input to change the carbon content significantly. With simple tools, the best you can do is provide the fastest quench possible to get maximum hardness. That means a lot agitation when quenching. If you are quenching with water, use a garden hose with a high pressure stream (sweeper nozzle), or vigorous shaking in the oil. For chisels or wear edges, it probably would be better suited to quench in agitated water and then tempering back some of the hardness (straw color to brown).

 

[Pennsylvania Weldor]

Just a quick input. Tempering is not a hardening process. Tempering is used to provide some ductility (softer) to something that has been hardened.

As far as using carburizing flames and oil quenches, that will not help. you are not at high enough temperatures and you don't have enough carbon input to change the carbon content significantly. With simple tools, the best you can do is provide the fastest quench possible to get maximum hardness. That means a lot agitation when quenching. If you are quenching with water, use a garden hose with a high pressure stream (sweeper nozzle), or vigorous shaking in the oil. For chisels or wear edges, it probably would be better suited to quench in agitated water and then tempering back some of the hardness (straw color to brown).

Totally disagree with your comment about carburizing flame and oil quenches. Ever heard of Carburizing Compound? It's used in a heat treatment process. If one doesn't have that compound or a heat treatment furnace, the flame and oil quench DOES work. Been doing it for over 40 years now. Rapidly cooling with water can and does result in failed metal because it's now brittle. As far as being at a high enough temperature, remember I said I was resharpening cold chisels (not attempting to harden softer steels) The chisel's are already high carbon steels, I'm just keeping them tough with my method.

 

[kid_108]

I am very familiar with carburizing compounds, and I also have very good understanding of the need for carbon potential and time required for carbon diffusion into austenite. What you are calling pores are actually interstitial sites for carbon diffusion, and yes the interstitial sites in austenite are larger than they are for ferrite, but you will never pull carbon out of oil into the interstitial sites. The reason is that in less than a second after you place the part in the oil, the steel's surface will already be converted back to ferrite, and it is physically impossible to dissolve carbon into ferrite (this phenomena is the reason why you can heat treat steel by quenching in the first place). As far as carburizing compounds are concerned, they need to work at very high temperatures because carbon diffusion requires several factors, including carbon potential and high temperatures.

There are heat treating processes that use carburizing atmospheres instead of compounds to re-carb steel (I re-carbed 10,000 parts this way last year). But the furnaces are sealed to prevent reaction with outside air. And, yes there is excess carbon in a carburizing flame, but you are losing the excess carbon because it is reacting with oxygen in the air before it has any chance to diffuse into the steel. (Likewise, I doubt that you are at high enough temperatures (>1800 degrees) to allow any diffusion -- in fact, most backyard heat treaters use a magnet (loss of magnetism) to tell them when to quench, when in reality, loss of magnetism is about 300 degrees to cold to be effective).

Agreed that rapid quenching in water will cause the steel to be brittle, that is why I suggested that it be tempered. It should be tempered right after being quenched, and before being used. Nobody here is talking about using tool steels, even your cold chisels are not high alloyed tool steels; so water quenching and tempering is not a problem.

I didn't try to knock your 40 years with my post, I was only trying to give my 35 years worth.