Martensitic steels machining

Without these advances in hard, strong materials based on abundant, and therefore low-cost iron ore, there could have been no industrial revolution in the nineteenth century. Long bridges, sky-scraper buildings, steamships, railways, and more, needed pearlitic steel (low carbon) for their construction. Efficient steam engines, internal combustion engines, turbines, locomotives, various kinds of machine tools, and the like, became effective only when key components of them could be constructed of martensitic steels (medium carbon). 

Martensitic Stainless Steels. The martensitic stainless steels have somewhat higher carbon contents than the ferritic grades for the equivalent chromium level and are therefore subject to the austenite—martensite transformation on heating and quenching. These steels can be hardened significantly. The higher carbon martensitic types, eg, 420 and 440, are typical cutiery compositions, whereas the lower carbon grades are used for special tools, dies, and machine parts and equipment subject to combined abrasion and mild corrosion. 

Martensitic chromium steels (AISI 400 series) contain no (or very little) nickel, and the chromium content is typically about 12%. These steels can undergo the a-Fe/7-Fe transition at about 1050 °C and so can be heat-treated for improved mechanical properties, much as can ordinary carbon steels. Since they have the a-Fe structure at ambient temperatures, they are ferromagnetic in ordinary service. Examples are type 410 (11.5-13.5% Cr), which is used for turbine blades, and type 416 (12-14% Cr with minor amounts of Se, Mo or Zr), which has good machinability. 

Ferritic stainless steels commonly contain 17% Cr and <0.12% C. Such steels are used in household appliances (e.g. washing machines and dishwashers) and in vehicle trim. Increasing the carbon content of ferritic stainless steels results in the formation of martensitic stainless... 

Martensite is formed when steel with a carbon content above 0.2 % is rapidly cooled from the austenite temperature range to a temperature below the martensite starting temperature. Due to the prompt cooling, the carbon dissolved in austenite is forced to remain dissolved in the mixed crystal. Martensite has a fine-acicular, very hard, and brittle microstmcture which causes increased abrasive wear and high mechanical and thermal stresses during machining. 

High-alloyed steels with a martensitic microstructure show machining results, which heavily depend on the workmaterial hardness and thus on the applied heat treatment. However, hardened and tempered martensitic stainless steels can be machined relatively well with suitable cutting parameters, tool materials, and coating systems, respectively. The dominant failure modes when using coated carbide tools for cutting hardened... 

Kumar AS, Durai AR, Somakumar T (2006) The effect of tool wear on tool life of alumina-based ceramic cutting tools while machining hardened martensitic stainless steel. J Mater Process Technol 173(2) 151-156... 

The machinability of martensitic stainless steels decreases with in creasing carbon content because of the higher amount of chromium carbides. Austenitic stainless steels have a high work-hardenabihty which causes a cold deformation of the surface during machining, which decreases the machinability. 

Alloy development for suction roll shells has barely kept pace with the ever-increasing performance demands and white water corrosivity in paper machines. Bronze, martensitic stainless steels, early-generation duplex stainless steels, precipitation hardening stainless steels, and later-generation duplex stainless steels have all been used for the manufacture of suction roll shells. Today, the preferred materials for suction roll shells in severe service are duplex... 

For passivation treatments other than for scale removal, less aggressive acid solutions are used. The purpose of these treatments is to remove any contaminants that may be on the component s surface that could prevent the formation of the oxide layer locally. The most common contaminant is embedded or free iron particles from forming or machining tools. A10% nitric add solution is effective in removing free iron. For martensitic, ferretic, and predpitation-hardening grades, a nitric acid solution inhibited with sodium dichromate is used so as not to attack the stainless steel too aggressively. 

Type 416 stainless steel is a low-carbon-class martensitic alloy, a free-machining variation of type 410 stainless steel. The chemical composition is shown in Table 9.4. It has a maximum continuous operating temperature of 1250°F (675°C) and an intermittent maximum operating temperature of 400°F (760°C). 

---