Comparison of hardness between carbon steel, stainless steel, and alloy steel, as well as considerations for valve stem material selection
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This post was last edited by cynthiarry on 2016-9-28 at 11:26. Hello everyone, I have a question. Generally, stainless steel bolts are considered low-strength bolts, while 35CrMo alloy steel bolts are high-strength bolts. Since higher strength usually implies greater hardness, does that mean the hardness of alloy steel is greater than that of stainless steel? Generally, the stem of valves is made of 13Cr, which is a martensitic stainless steel. Stems require high hardness, and I wonder whether their hardness is greater than that of alloy steel? Additionally, material hardness is also related to carbon content; so does that mean carbon steel is harder than stainless steel? If so, why isn’t the valve stem made of carbon steel?
It is also necessary to take corrosion prevention, cost, and the issue of rusting when not in use for long periods into consideration
What about the hardness of various materials? Is it carbon steel > alloy steel > stainless steel?
It’s not possible to make generalizations; the carbon content in stainless steel varies, and corrosion needs to be taken into account for valve stems
Valve stems need to possess strength, toughness, and corrosion resistance; therefore, stainless steel with surface nitriding is generally used for such stems, combining the advantages of both properties to ensure strength, toughness, and corrosion resistance.
It means that corrosion also needs to be taken into account, not just hardness. But I still want to ask what the approximate order of strength and hardness is for these various materials?
With reference to the above, it should be added that some valve stems are of the exposed type; those that come into contact with the medium need to consider not only wear resistance but also corrosion resistance. As for hardness, it cannot be generalized – it is primarily determined by the chemical composition of the material, its heat treatment conditions, and its crystal structure.
We now have an approximate answer regarding the issue of hardness. Starting with bolts, carbon steel bolts are low-strength bolts, stainless steel bolts are medium-strength bolts, and alloy steel bolts are high-strength bolts; for more details, refer to the sections on bolts in ASME B16.5. Additionally, 13Cr is chosen for the valve stem mainly to address corrosion issues. Furthermore, although carbon steel has a higher carbon content than stainless steel, its hardness is not necessarily higher; for example, pipes made of grade 20 steel are less hard than 304 steel pipes.