What Is Molybdenum Steel?

Molybdenum steel is a low-alloy strong hot steel. Among them, molybdenum is one of the important metal elements to improve the thermal strength of hot-strength steels. ^[1]

In the 1930s, in order to solve the problem of improving the steam parameters of power equipment and extensive exploration of material problems, it was found that molybdenum was an improved hot

More detailed performance data for steel are shown in Figure 2.

First, molybdenum is an element soluble in alpha ferrite. It increases the activation energy of self-diffusion of iron, and improves the recovery of iron.

Molybdenum steel and chrome molybdenum steel are suitable for working at high temperature. Therefore, it is widely used to manufacture headers and ducts of high-pressure boilers. Because of its high strength and good weldability, chrome-molybdenum steel has been widely used in aircraft structures in the past, but recently it has been completely replaced by cheaper chrome-manganese-silicon steel that is not far from it in all respects. ^[2]

Molybdenum steel effect

In molybdenum steel and chromium-molybdenum steel, molybdenum is an element that forms carbides. Molybdenum in steel is basically

or

Carbides are present, and only a part is dissolved in the fat granules. It can increase the strength of steel without reducing the ductility and toughness. At high temperatures, molybdenum can maintain the strength of the steel and reduce the tendency to creep. In addition, molybdenum can increase the hardening depth of the steel and refine the grain of the steel. In chromium steel and other alloy steels, molybdenum can prevent the steel from tempering brittleness. Chromium can increase the creep limit and thermal stability of molybdenum steel, but chromium often forms a thin layer of refractory oxide on the surface of the steel, making chromium-molybdenum steel less weldable than molybdenum steel. ^[2]

Molybdenum steel welding method

The method of welding molybdenum steel is similar to that of ordinary carbon steel. For gas welding, use neutral flame or slight carbonized flame without flux, usually use H08A brand welding wire. However, when strength requirements are high, H18CrMoA grade welding wire is used.

In general, preheating is not required before welding. However, if the welding site temperature is lower than zero, it should be preheated to 149 ~ 430 ° C according to the thickness of the material before welding. See Figure 7 for its welding specifications.

Figure 7

During welding, the liquid metal in the molten pool should be kept as thick as possible, which can reduce overheating and loss of chromium and molybdenum. During the welding process, the welder should try his best to reduce the interruption in the middle, so as not to cause many weld joints and the surface is not smooth and beautiful.

At the end of welding, the welding speed should be fast, the angle between the welding tip and the workpiece should be small, fill the molten pool, and then slowly leave the flame away from the molten metal. Formation of bubbles.

When welding is stopped and then re-welded, in order to avoid cracks, the weld must be uniformly heated to below 250 ~ 300 ° C with a flame. ^[2]

Post-weld heat treatment of molybdenum steel

In order to obtain higher strength (47 ~ 145 kg / mm2) and meet the requirements for use, the following treatments can be performed after welding.

1. The weldment is heated to 900 ~ 950 ° C and slowly cooled with the furnace.
2. It is heated to 845 ~ 900 ° C and quenched in oil.
5. It is then heated to 500 ~ 580 ° C, then cooled in still air and tempered. ^[2]