Why high carbon steel is more difficult to weld

High-carbon steel refers to w (C) higher than 0.6% of carbon steel, which has a greater tendency to harden than medium carbon steel, and the formation of high-carbon martensite, more sensitive to the formation of cold cracks.

At the same time, the martensite organization formed in the welded heat-affected zone, hard and brittle properties, resulting in a significant reduction in the plasticity and toughness of the joint, so the weldability of high-carbon steel is quite poor, and must take special welding process to ensure the performance of the joint.

Therefore, in the welded structure, generally rarely used. High-carbon steel is mainly used for machine parts that require high hardness and wear resistance, such as shafts, large gears and couplings, etc. .

To save steel and simplify the machining process, these machine parts are also often combined in welded structures. In heavy machine manufacturing, welding of high-carbon steel components is also encountered.

When developing the welding process of high-carbon steel weldments, a comprehensive analysis of the various welding defects that may arise and the corresponding welding process measures should be taken.

1 Weldability of high carbon steel

1.1 Welding method

High-carbon steel is mainly used for high hardness and high wear resistance structures, so the main welding methods are electrode arc welding, brazing and submerged arc welding.

1.2 Welding material

High-carbon steel welding generally does not require the strength of the joint and the base material. Welding electrode arc welding is generally used to remove sulfur capacity, low hydrogen content of diffusion of the deposited metal, good toughness of low-hydrogen type welding rod. In the requirements of the weld metal and the parent material and other strength, the corresponding level of low-hydrogen electrode should be used; in the weld metal and the parent material and other strength, the strength level should be used below the parent material of low-hydrogen electrode, remember not to choose the strength level than the parent material high electrode. If the base material is not allowed to preheat when welding, in order to prevent cold cracking in the heat-affected zone, austenitic stainless steel welding rod can be used to obtain good plasticity and cracking resistance austenitic organization.

1.3 Bevel preparation

In order to limit the mass fraction of carbon in the weld metal, the fusion ratio should be reduced, so the general use of U- or V-shaped bevels when welding, and pay attention to the bevel and the bevel on both sides of the 20mm range of oil, rust and other treatment clean.

1.4 Preheating

Structural steel electrode welding, welding must be preheated before, preheating temperature control at 250 ℃ ~ 350 ℃.

1.5 Interlayer processing

Multi-layer multi-channel welding, the first welding using small diameter welding rod, small current welding. Generally place the workpiece in a semi-standing welding or use the welding rod lateral swing, in order to make the entire heat-affected zone of the parent material are heated in a short period of time, in order to obtain preheating and insulation effect.

1.6 Post-weld heat treatment

Immediately after welding, the workpiece is placed in a heating furnace and held at 650°C for stress relief annealing.

2 high-carbon steel welding defects and preventive measures

Due to the high-carbon steel hardening tendency is very large, in the welding prone to hot cracking and cold cracking.

2.1 Preventive measures for thermal cracking

(1) control the chemical composition of the weld, strict control of sulfur and phosphorus content, and properly increase the amount of manganese to improve the weld organization and reduce segregation.

2) Control the shape of the weld section, the width-to-depth ratio should be slightly larger to avoid the deviation of the weld center.

(3) for the rigid welded parts, should choose the appropriate welding parameters, the appropriate welding order and direction.

4) Preheating and slow cooling measures are taken when necessary to prevent the generation of thermal cracks.

(5) improve the alkalinity of the electrode or flux to reduce the impurity content of the weld and improve the degree of segregation.

2.2 Cold cracking prevention measures 

1) Preheating before welding and slow cooling after welding not only reduces the hardness and brittleness of the heat-affected zone, but also accelerates the outward diffusion of hydrogen in the weld.

2) Selecting the appropriate welding measures.

3）Adopt suitable assembly and welding sequence to reduce the constraint stress of the welded joint and improve the stress state of the welded parts.

4) Select suitable welding materials, dry the welding rod and flux before welding, and make it available as you go.

5) Before welding, the water, rust and other dirt on the base metal surface around the bevel should be carefully removed to reduce the content of diffused hydrogen in the weld.

6) Hydrogen treatment should be carried out immediately before welding, so that hydrogen can fully escape from the welded joint.

7）Annealing treatment of stress relief should be carried out immediately after welding to promote the diffusion of hydrogen in the weld seam outward