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Martensitic Stainless Steel
Description of material
ACMV is a Chromium- Molybdenum –Vanadium Martensitic steel used for heavy duty inlet valves.
Intake valves for gasoline and diesel internal combustion engines with operating temperatures up to 700° C. In addition, ACMV could be used in low stressed exhaust valves but it’s useful to point out that, as with all martensitic alloy steel grades, it shows an increase of elongation with a rapid decrease in strength as temperature increases. ACMV offers good hot mechanical properties up to 680°C. This grade has a good wear resistance.
ACMV has a good resistance to oxidation and hot corrosion in combustion products environments that form inside diesel and gasoline engines up to 700°C providing better results if compared to VAL5.
ACMV has a good machinability in the hardened and tempered condition. Cold shearing, band or circular sawing and grinding wheel cutting can be used. In the case of turning, its machinability is approximately comparable to hardened and tempered alloy steels and could be only improved with a right choice of cutting fluids, tools and machining parameters.
ACMV is primarily used for monolithic, not for bimetallic valves. Others grades, such as VAL5 or VAL5M, should be used for this last application. In any case, in solid state joining such as Friction Welding, ACMV provides a quality bond line. When friction welded with head valve pieces of austenitic or Ni alloy grades, a stress relieving or tempering of the valve must be done in order to soften the martensitic structure of HAZ and bond line.
ACMV has a good hot plasticity and is suitable for processing by hot extrusion or by upsetting with electric resistance heating. However, overheating must be always avoided. The choice of hot working temperature and process parameters must always evaluate both the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top end of the range during the extrusion and forging process, could generate internal bursts of stems of valves. The same care should be carried out for electrical upsetting process in order to maintain the right geometry of the bulb and to avoid hot cracks and internal shortness of the head valve. A low or medium temperature range of hot working is recommended. In case of close die forging of large size valves, a slow cooling after forging is mandatory and an evaluation of the influence of the retained austenite amount should be considered.
ACMV must be always hardened and tempered in order to increase its mechanical properties and a stress relieving, in case of friction welding, must be always done. The use ACMV in the annealed condition must be avoided.