High carbon steel refers to the carbon content of more than 0.6% of the steel, it is mainly used to require high hardness and wear resistance of machine parts, such as rotating shaft, gear, bearing and coupling. It has a greater tendency to harden than medium carbon steel, and forms high carbon martensite, which is more sensitive to the formation of cold crack. At the same time, the martensite structure formed in the welding heat affected zone has a high hardenability, which leads to the decrease of the plasticity and toughness of the joint and poor weldability. Therefore, special welding technology must be adopted to ensure joint performance. When making the welding process of high carbon steel welding parts, all possible welding defects should be taken into full consideration and the corresponding welding process should be adopted.
The welding process of high carbon steel
take the 1084 steel for example. Welding method: High carbon steel is mainly used for structural purposes where requiring high hardness and high wear resistance. Arc welding, brazing welding and submerged arc welding are the main welding methods.
Welding material: High carbon steel welding generally does not require joint and base metal strength. Electrode arc welding is generally used to have a strong ability to remove sulfur, low hydrogen content of the deposited metal diffusion, good toughness of low hydrogen electrode, rather than the strength level is higher than the base metal electrode. When the strength of weld metal and base metal is required, the corresponding low hydrogen electrode should be selected;
If the base metal is not allowed to preheat during welding, in order to prevent cold cracks in the heat-affected zone, Austenitic stainless steel electrode can be selected to obtain good plasticity and strong anti-crack Austenitic organization.
Groove: In order to limit the carbon content in the weld metal, the fusion ratio should be reduced. U-shaped or V-shaped grooves are generally adopted, and the oil and rust in the range of 20mm on both sides of the groove and the groove are cleaned.
Preheating: Structural steel electrode must be preheated before welding, preheating temperature is controlled at 250℃ ~ 350℃.
Interlayer processing: Multi-layer multi-pass welding, the first welding using small diameter electrode, small current welding. Generally, the workpiece is placed in semi-vertical welding or the use of welding rod transverse swing, so that the whole base metal heat-affected zone is heated in a short time for preheating and insulation effect.
Post-welding heat treatment: Immediately after welding, the workpiece is placed in a heating furnace for heat preservation and stress relief annealing at 650℃.
High carbon steel has a great tendency to harden, and it is easy to appear hot crack and cold crack during welding. How do you prevent thermal cracking?
1) Control the chemical composition of the weld. Strictly control the content of sulfur and phosphorus, and appropriately increase the content of manganese to improve the weld microstructure and reduce segregation.
2) Control the weld section shape, the width to depth ratio should be slightly larger to avoid segregation in the weld center.
3) For welding parts with high rigidity, appropriate welding parameters, welding sequence and direction should be selected.
4) Preheat and slow cooling measures should be taken when necessary to prevent the generation of hot cracks.
5) Improve the basicity of welding electrode or flux to reduce the content of impurities in the weld and improve the degree of segregation.
How to prevent cold cracks of high carbon steel?
1) Preheating before welding and slow cooling after welding can not only reduce the hardness and brittleness of the heat-affected zone but also accelerate the hydrogen diffusion in the weld.
2) Select appropriate welding measures, assembly and welding sequence to reduce the constrained stress of the welded joint and improve the stress state of the weldment.
3) Choose the right welding materials, dry the electrode and flux before welding, and use them at will.
4) Before welding, the water, rust and other impurities on the surface of the base metal around the groove should be carefully removed to reduce the content of diffused hydrogen in the weld.
5) Stress relief annealing treatment should be carried out immediately after welding to promote the outward diffusion of hydrogen in the weld.