Weld Crack

A. Weld Crack

There are two kinds of weld cracks. One is low-temperature cracks that occur at a comparatively low temperature and the other is high-temperature cracks that occur during melting and solidification. Low-temperature cracks include underbead cracks, root cracks, and transverse micro-cracks in the weld metal. Though they rarely occur in low-carbon steel, they easily do in high-carbon steel, so even low alloy steels call for special caution. They are caused due to chemical composition (especially the carbon equivalent) in view of material, therefore, weld cracks are associated with the extent of hardening and the base metal's  heat treatment, resulted from welding. Weld heat input, hydrogen embrittlement, residual stress, and stress concentration also affect weld cracks. For this reason, it is important to give proper preheating and postheating

treatments, use low-hydrogen electrodes, and consider constraint conditions for welding.

 

On the other hand, as the carbon content increases, high-temperature cracks tend to occur easily. They typically occur at starting points of beads and in the crater zone.

 

Types of weld cracks

Low-temperature cracking is a type of delayed fracture that occurs when hydrogen diffusing from the weld metal acts on the hardened zone, to which strong restraint stress is being applied as a result of the fact that thermal contraction during welding is prevented. Therefore, the prevention of low-temperature cracking requires comprehensive consideration of three parameters: � weld hardenability of base metal, � intensity of restraint and � the amount of diffusible hydrogen. The weld cracking parameter (Pc) shown below is commonly used to predict low-temperature cracking.

 

 

Fig. shows the relationship between the parameters in P_c and low-temperature cracking behavior. We can see that precise prediction of low-temperature cracking behavior is made possible by the P_c formula.

 

Relationship between P_c and cross section crack ratio

In actual welding, preheating and post-heating are performed to prevent weld cracking. JIS Z3138 "Oblique y-groove weld cracking test" is commonly used to determine the temperature for the above. The preheating temperature, which is determined by this test, and the PC formula have a good relationship, as shown in below Fig., and its mathematic representation is as follows.

 

 

Relationship between P_c and preheating temperature

T0 = 1440 P_c-392

T0: Preheating temperature for prevention of cracking (°C)

 

Generally speaking, the reduction of preheating and post-heating with consideration for the actual construction period and cost aspects is a major need of customers. The way to meet this need is to reduce the PCM of the steel, i.e., the amount of carbon and other alloying elements added, while satisfying the various characteristics that are required. At present, the technique used as the means to achieve the above is TMCP, which makes it possible to achieve high strength with a small amount of alloying elements. TMCP is applied in a wide variety of fields in which the reduction of preheating and post-heating is demanded.

 

B. Notch Toughness

Notch toughness can be substantially controlled by proper welding work. In view of materials, it is adversely affected by carbon-equivalent dependent weld hardening and embrittlement that are caused by entry of hydrogen, nitrogen and oxygen. Moreover, some steel plates including appropriate amounts of V, Al, Ni, etc. improve in toughness to a considerable extent due to increasingly refined grain.

 

 

Source: nssmc.com