One Alloy. Many Uses
As we previously mentioned, the stainless steel that Harry Brearley discovered was what came to be known as type 420, or what we call a martensitic stainless steel. Or a hardenable stainless steel that may be hardened and tempered, as are normal carbon or alloy steels. There are also 400 series stainless steels that are not hardenable, namely ferritic stainless steels.
The 400 series are found in myriad applications, including cutlery, dental and surgical instruments, forged fasteners, forged high-temperature bolts, and in the case of the higher chromium types, parts where scaling resistance is required.
The basic type 420 has 13% chromium and a minimum of 0.15% carbon. It may be heat treated to be used as cutlery and other cutting instruments. Its corrosion resistance is somewhat limited, but adequate for fairly mild environments. With time the basic 13% chromium grade was modified to produce steels with lower or higher carbon, and additions of nickel – for hardenability – and other elements such as molybdenum, vanadium and tungsten, again for hardenability plus increases in mechanical properties and heat resistance. Types 440 A and 440C, for example, move to 0.7 to over 1% carbon, and can reach Rc 56 to 60 when heat treated. Care must be taken when forging these high-carbon 400 series, and although good for wear resistance, and resistance to fairly mild corrosive environments, they are not suitable for use at elevated temperatures.
A Delicate Process For a Unique Material.
Great care should be taken when forging and heat treating the 400 series hardenable grades. The higher carbon grades are best preheated prior to bringing up to forging temperature, and should be brought back to room temperature following forging, prior to heat treatment. Soaking for any significant length of time at 750/1050 ºF will result in loss of corrosion resistance and impact strength, due to carbide precipitation. In any event, the services of a trustworthy heat treater might best be called upon.
The ferritic 400 grades are characterized by lower carbon, and apart from types 440A and 440C, higher chromium than the hardenable grades. The ferritic grades are mostly resistant to fairly mild corrosive environments, and the 440 grades are resistant to scaling at high temperatures. Type 430 is resistant to nitric acid, and as such is used in heat exchangers, condensers and piping for this acid.
Each 400 series grade of steel was originally designed to fill a certain need, and the steels need to be forged and heat treated carefully. Certain 400 grades are susceptible to carbide precipitation and attendant loss of corrosion resistance and impact strength. AMFG has fabrication data on all grades supplied for various industry uses of custom forged parts and seamless rolled rings.
