Before the roast begins let me explain myself. For years I have let students at my university lab make small knives and caseharden parts using bone and charcoal. Invariably the question comes up each time whether they could just use mild steel for a knife blade and caseharden it. I have always said that it wouldn't make a very good knife blade. But now I am questioning some aspects of that.
Looking at Wick's very nice Penny knife in the recent thread got me thinking. These things were mass produced in England and, perhaps, the colonies and were sold very cheaply (hence the name).
From my understanding of metallurgy from this era wrought iron (in progressively finer grades) would have been the cheapest of the ferous metals available, followed than by blister steel, shear steel, and eventually crucible steel. With each phase beginning with blister steel carbon distribution would have gotten progressevely more uniform and the structure would have become finer.
Blister steel got its name from the cementation process where large steel bars were packed in carbon (charcoal) and heated past critical for hours/days allowing the carbon to absorb into the bars. Following this the bars would have been covered with small patches resembling blisters giving the basis for the name.
At this stage I presume the material could have been used for some things as is, or it would have been further refined by forging it out, working the carbon deeper and more uniformly into the steel. Once the thickness was reduced the bard would cut, and forge welded back on each other to gain thickness and the process completed. At this stage the material was referred to as shear steel, or double shear. The shear implied the suitability of this material for cutting instruments.
So my question is this. On a very cheap knife, would manufactures have taken the time to go to the next stage of the process and use shear steel which would have been more costly to produce. Even blister steel as mentioned above involved very large bars of iron and sometimes days in the furnace to convert to blister steel. Why would they not simply use a good grade of wrought iron (which was to my understanding available in different grades), shape the blade to almost finished dimensions (including drilling the pivot hole), and then carburize a quantity of these.
In our lab the carburizing process that we use can provide very deep penetration. I often have students wrap the parts in soft iron wire that is about 1/16" in diameter to improve the color pattern. After the quench we find that the carbon seems to completely penetrate through the wire suggesting that even at the short times (2 hours) that the casehardening takes that the carbon penetration is sufficient to make this wire brittle to the point of being able to break apart with our fingers. On a thin pocket knife blade I have no doubt that the carbon would penetrate completely through even the thickest cross section in a relatively short period of time (4-5 hours). Even if it didn't penetrate through say the spine this would not be all that undesirable as the edge would certainly be completely carburized for a good distance back. This could provide the same effect as a laminated steel blade. Following the carburizing process the blades could be allowed to cool down slowly in the charcoal effectively annealing them and minor adjustments, clean up could be performed followed by a conventional hardening and tempering operation.
So after my somewhat long winded explanation/defense, where do we stand? Is it possible to make a knife blade from mild steel? Would this have been a process used in early colonial knife production for cheap blades? I am not in any way suggesting this is a good idea for a modern high performance knife and the material wouldn't come even close to our most basic tool steels available today, but would it have been, in a historical context, a likely occurrence a historical possibility? Am I off my rocker?
Alex Johnson
Looking at Wick's very nice Penny knife in the recent thread got me thinking. These things were mass produced in England and, perhaps, the colonies and were sold very cheaply (hence the name).
From my understanding of metallurgy from this era wrought iron (in progressively finer grades) would have been the cheapest of the ferous metals available, followed than by blister steel, shear steel, and eventually crucible steel. With each phase beginning with blister steel carbon distribution would have gotten progressevely more uniform and the structure would have become finer.
Blister steel got its name from the cementation process where large steel bars were packed in carbon (charcoal) and heated past critical for hours/days allowing the carbon to absorb into the bars. Following this the bars would have been covered with small patches resembling blisters giving the basis for the name.
At this stage I presume the material could have been used for some things as is, or it would have been further refined by forging it out, working the carbon deeper and more uniformly into the steel. Once the thickness was reduced the bard would cut, and forge welded back on each other to gain thickness and the process completed. At this stage the material was referred to as shear steel, or double shear. The shear implied the suitability of this material for cutting instruments.
So my question is this. On a very cheap knife, would manufactures have taken the time to go to the next stage of the process and use shear steel which would have been more costly to produce. Even blister steel as mentioned above involved very large bars of iron and sometimes days in the furnace to convert to blister steel. Why would they not simply use a good grade of wrought iron (which was to my understanding available in different grades), shape the blade to almost finished dimensions (including drilling the pivot hole), and then carburize a quantity of these.
In our lab the carburizing process that we use can provide very deep penetration. I often have students wrap the parts in soft iron wire that is about 1/16" in diameter to improve the color pattern. After the quench we find that the carbon seems to completely penetrate through the wire suggesting that even at the short times (2 hours) that the casehardening takes that the carbon penetration is sufficient to make this wire brittle to the point of being able to break apart with our fingers. On a thin pocket knife blade I have no doubt that the carbon would penetrate completely through even the thickest cross section in a relatively short period of time (4-5 hours). Even if it didn't penetrate through say the spine this would not be all that undesirable as the edge would certainly be completely carburized for a good distance back. This could provide the same effect as a laminated steel blade. Following the carburizing process the blades could be allowed to cool down slowly in the charcoal effectively annealing them and minor adjustments, clean up could be performed followed by a conventional hardening and tempering operation.
So after my somewhat long winded explanation/defense, where do we stand? Is it possible to make a knife blade from mild steel? Would this have been a process used in early colonial knife production for cheap blades? I am not in any way suggesting this is a good idea for a modern high performance knife and the material wouldn't come even close to our most basic tool steels available today, but would it have been, in a historical context, a likely occurrence a historical possibility? Am I off my rocker?
Alex Johnson
