Hello everyone,
My mind is occupied with an issue. I wanted to discuss that with you.
As you know, in case hardening, carbon atoms diffuse in the subsurface of the steel and lock there when the steel gets quenched afterward. The resulting martensitic structure hardens the surface, so we benefit from the hardened surface + toughness of the core.
But consider the situation when high-temperature alloys are used in the petrochemical sector, carbon atoms liberated from the hydrocarbons in the coils, diffuse inside the structure and then combine with the metallic atoms and form carbides.
The problem is these carbides. What is the difference between case hardening and carburization in the petrochemicals sector that the first is an advantage wheras the second is a catastrophic event?
What do these carbides do with the structure? Where exactly do they form? at grain boundaries or inside grains? how do they induce corrosion? what type of corrosion occur? I hope you participate in this discussion. Thank you in advance
Indeed this story has long been known and perfectly studied. In a nutshell and somewhat simplified, it looks like this:
Corrosion-resistant steels are corrosion-resistant because they contain a lot (> 12%) of dissolved chromium. Chromium in these quantities forms a dense Cr2O3 layer (passive layer) on the steel surface, which protects the steel from wet corrosion. During carburization, chemically active chromium first reacts with the carbon and forms carbides, which are present in the form of crystals in the steel structure. The proportion of dissolved chromium drops below the critical 12% and that is no longer enough for the formation of a continuous passive layer, steel is no longer corrosion-resistant.
Thank you Vadim,
As I realized from what you stated, we should not case hardened the material, in which there is even trace of chromium. Because chromium may combine with carbon and form carbides through case hardening. is that right?
If you want your chromium to be in solid solution form (corrosion resistant stainless steels) then the carbon content needs to be tightly controlled to keep enough chromium in the solution. However, there are other steels with chromium, where chromium is mainly present in carbide form and is in this form desirable, for example HSS steels. Such steels are not resistant to corrosion, but are very hard and wear-resistant.
Thank you Vadim for continuing this discussion, It is very valuable
Consider the situation where you have petrochemical coils made of corrosion-resistant stainless steel and in an environment of active carbon. (The case study of hydrocarbon gas crackers)
What is happening? carbon diffuses into the steel since chromium oxide is not stable above 850°C. Then carbon combines with chromium and form carbides, which is a disaster and undesirable but I do not know why?
You mentioned that the desirable form of carbide exists in HSS steels.
My question is what is the difference between the desirable form and the undesirable form of carbides? why does one kind of them induce corrosion as long as both form at grain boundaries?
Dear Arezoo,
You did not understand correctly. The formation of chromium carbides during carburation does not impair the properties of high-speed steels, since they do not require high corrosion resistance, but only strength and hardness are required. It is a completely different matter - corrosion-resistant steels, in which the formation of chromium carbides during carburation causes a decrease in the amount of dissolved chromium. In this case, it does not matter at all whether carbides are formed at the grain boundaries or in some other way, it is only important that the chromium in the solid solution becomes less than the critical minimum.
Dear Vadim,
Thank you for the clarification to distinguish between unavoidable carburization and deliberate carburation.
As you know, in carburization, carbon takes up the dissolved chromium and forms carbide, this issue results in chromium depletion in the substrate till the moment that the depleted area of the chromium deficits strength and finally the mechanical properties like strength and hardness of the surface deteriorates.
Do you think that the same might happen in carburation?
The other question for me is how carbides in corrosion-resistant alloys induce corrosion. Corrosion means that an electrochemical reaction happens between the anode and cathode. Does that mean the formed carbide at grain boundaries through grain boundaries makes grain boundaries more anodic or cathodic with respect to the grains inside? If we consider this situation as corrosion then where is the electrolyte to complete the corrosion reaction?
Dear Arezoo
The carbides create heterogeneity in the metallic structure by precisely occupying the grain boundaries. this will cause intergranular corrosion.
I hope I brought you a few things.
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