Common Metal Materials Welding

I. Welding of low alloy structural steels

1. Welding characteristics

The carbon content of low alloy structural steels is very low, increasing carbon content increases the hardness and strength and improves hardenability. Weldability deteriorates with increasing strength.

(1) Low alloy structural steels with σs=300~400MPa have good weldability.

(a) When welding at normal temperature, treat it as welding mild steel;

(b) When welding at low temperature or welding structural steel with large thickness and rigidity, the current should be increased, the welding speed should be reduced, alkaline electrodes should be selected and preheated;

(c) When welding pressure vessels, pay attention to annealing after welding to eliminate stresses.

(2) Low alloy structural steels with σs ≥ 450 MPa have poor weldability.

(a) Preheating before welding.

(b) Using high current and low welding speed during welding.

 (c) Tempering after welding.

(d) Drying the electrode or flux and carefully cleaning the welded parts.

II. Welding of cast iron

Cast iron is very poor weldability and is generally only performed to repair certain casting defects.

1. Welding properties of cast iron

(1) White tissue is easily produced.

(2) Easy to produce cracks.

(3) It is easy to produce pores and slag inclusions.

2. Repair welding process

(1) Hot repair welding

Before repairing welding,preheating the welding parts partially or entirely to 600~700maintain it during the welding process, and cool it slowly after welding.

(2) Cold repair welding

Welding guns are only preheated to below 400°C.

The cold repair welding mainly depends on the chemical composition of the welding wire to avoid whiteout tissue and cracks.

III. Stainless steel welding

1. Austenitic stainless steels: high content of Cr and Ni elements, low C content

(1) Weldability is good, generally process measures are not required during welding.

(2) When selecting welding rods, wires and fluxes make sure that the weld metal has the same composition as the base metal.

(3) Use a small current, fast non-swing welding, and increase the cooling rate after welding.

(4) Surfaces in contact with corrosive media should be welded last.

2. Ferritic stainless steel

(1) Ferrite grains in the heat-affected zone are easily coarsened by overheating.

(2) The preheating temperature before welding should be below 150°C.

(3) Use small current, fast welding processes to reduce the tendency to grain coarsening.

3. Martensitic stainless steel

(1) It increases hardness and strength and cold cracks are easy to occur.

(2) Preheating temperature of 200~400°C before welding.

(3) Post-heating of welding parts is required, otherwise austenitic stainless steel filler metal should be used.

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