REASON TO USE ALLOY STEEL
In this post we explain the term of alloy steel and the reason why we use the alloy steel for special purpose.
What is alloy steel?
Alloy steel are those material that having alloying element other than carbon. Total alloying element is less than 10% typically alloying element are chromium, nickel, molybdenum, vanadium and niobium.
Why alloy steel is required?
As we know all material loses their strength as temperature increases. Also use of carbon steel is limited to 343 degree C. But we have processes operating at temperatures as high 550 degree C. We need higher metallurgies to handle these temperature and other corrosive conditions.
Hence we need alloy steel.
Type of alloy steel generally used
P11, P22, P5, P7, P9
Alloy steel can be used for higher operating temperature as they can retain their strength at higher temperature for example Steam service P22 is used for 100 barg super heater steam piping at 510 degree C.
Alloy steel generally used in petrochemical and refineries where sulphur bearing fluids are processed. Sulphur is an unavoidable impurity that comes out along with crude in the oil well.
Crude and petroleum product have sulphur to various extents and corrosivity of these sulphur compound in the crude and other petroleum products depends upon concentration and temperature.
Alloys steel exhibit corrosion resistance toward these sulphur compounds, higher the concentration and temperature, alloy with more chromium have to be used.
Alloy steel is also used in hydrogen services where the operating temperature is more than 250 degree C. As hydrogen is the smallest atom and hence have tendency to easily diffuse through the solid steel matrix. If operating temperature is more than 250 degree C then hydrogen reacts with carbon in steel to form of methane.
So carbon, basic strength provider to steel is lost in form of methane and hence strength of steel reduces. This gives rise to a phenomenon called hydrogen embrittlement. To avoid this alloy steel is used.
In case of alloy steel carbon reacts with chromium to form chromium carbide. The functions of carbon as well as carbide are same. Hence no methane formation reaction and hence no embrittlement.