Welding 4130 or 4140 steel in the quench and tempered or case-hardened conditions without first annealing or normalizing the area to be welded is not advised. Choose a filler metal. A welding rod made of 42% zinc and 58% aluminum. It produces less than half the amount of nitrogen as bare steel but more than enough to make a good weld.
The best way to avoid poisoning yourself with hydrogen sulfide is to allow plenty of time between welding jobs and to use adequate ventilation. The smell of gas isn't always easy to detect when it's present in low levels. Three-fourths concentration of gas can cause symptoms in people who are sensitive to it. With higher concentrations, you might need a mask.
Gas welding uses a combination of heat and light to join metals together. The heat melts the metal on the surface where it contacts the flame or hot metal from the furnace, and the light activates the oxygen in the air to form oxide layers that bind the pieces of metal together. The main types of gas used for welding are carbon dioxide, acetylene, and helium. Carbon dioxide is the most common type of gas used because it's available and affordable. It's also known as dry gas. Acetylene is used mostly for welding heavy metals like iron and steel. It's the fuel component in acetylene lamps and open-flame torches.
4130, 4340, and 8630 are the most widely welded heat-treatable steels. This assures that following heat treatment, the weld metal will have the same strength and ductility as the underlying material. Welding 4130 or 4340 produces a metal with greater hardness and lower yield strength than the parent material. Therefore, these metals must be worked harden before welding to bring them up to the level of the surrounding material. Welding 8630 gives a metal with less hardness and higher yield strength than the parent material. Thus, 8630 can be welded more easily but requires careful workmanship to ensure that adequate fusion occurs between the joint partners.
Alloys containing greater than 1% nickel or copper should not be welded. These alloys tend to form brittle intermetallic compounds at the joint interface that cannot be dissolved by conventional heat treatments. Instead, they must be treated like stainless steel.
Welding heats the material being joined together. When the heat is removed, the materials cool quickly because they are relatively poor conductors of heat. The two pieces of steel must be kept separate during welding to prevent their melting together. Also, the filler material used for welding should not be thicker than 3/8 inch because thicker pieces of material take longer to fuse together.
TIG and MIG welding may be used on 4130 chrome-moly. This filler metal may produce welds that are similar to the strength of 4130. However, MIG welding may produce some areas with lower strength because of the lack of filler metal in those areas.
The best way to ensure a strong weld is to adhere to the recommended procedures. For example, MIG welding 4130 chrome-moly should be done with shielding gas or flux cored wire. This will help prevent low strength areas in the weldment.
Also, make sure you clean the parts being welded thoroughly before starting. This will remove any debris that could affect the quality of the weld.
Finally, test all welds for defects before sending them out for heat treatment.