company news about Cracking after heat treatment? Hidden Trap of Welding Reheat Crack

01 Definitions

High-strength steels and superalloys containing precipitation-strengthening elements (Al, Ti, Nb) – including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-strengthened superalloys, and certain austenitic stainless steels – may develop stress relief cracks (SR cracks) during post-weld heat treatment, even though no cracks were initially present. Similarly, some welded structures may develop cracks during prolonged high-temperature service (e.g., 500–600°C). In engineering practice, these cracks – occurring during both stress relief treatment and service operation – are collectively termed reheating cracks.

02 Main characteristics of reheating cracks

(1) Reheat cracking only occurs in metal welds containing precipitation strengthening elements, and carbon steel and solid solution strengthening metal materials usually do not produce reheat cracking.

(2) There is a sensitive temperature range, which is related to the reheating temperature and time. The sensitive range of low alloy steel is about 500～700℃, and that of austenitic stainless steel and high temperature alloy steel is 700～900℃.

(3) The grain boundary of austenite coarse grain in the heat affected zone extends along the fusion line on the side of the base material, and the intergranular cracking is observed.

(4) There should be large residual stress and stress concentration in the welding area.

03 Factors affecting reheating crack

(1) The content of carbide-forming elements (Cr, Mo, V, Ti, Nb) has a significant effect. For example, the content of V in pearlite heat-resistant steel will greatly increase the sensitivity of SR crack.

(2) The heating speed and time affect the sensitive temperature range of different steel.

(3) The grain size has a significant effect on the reheat crack tendency, the larger the grain size, the higher the tendency.

(4) Differences in welding methods: The large heat input welding tends to cause grain coarsening, and the reheat cracking tendency of submerged arc welding is higher than that of electrode arc welding in the steel grades sensitive to grain growth, while the hardening tendency is higher in the steel grades with large hardening tendency.

04 Preventive measures against reheating cracks

(1) Select low strength matching welding materials;

(2) Control the cooling rate by using preheating or postheating;

(3) Avoid sensitive temperature range or shorten the residence time;

(4) Reduce residual stress and avoid stress concentration;

(5) Certain alloys (e.g., Incoloy 800HT designed for temperatures ≥538°C) require post-weld stabilization heat treatment;

(6) Nondestructive testing should be added to the materials with reheating crack tendency after heat treatment.

05 Reheating crack sensitive materials

15MnVR, 15MnNbR, 18MnMoNbR, 13MnMoNbR, 07MnCrMoVR, 07MnNiMoVDR, 17Cr1Mo1V, and Japanese CF-62 steel.

Note: Reheat cracks, which are hidden and can easily cause sudden accidents, may occur during heat treatment or service. Pressure vessel designers, manufacturers, and inspectors must assess reheat crack risks in advance and develop prevention and control plans.

Reheat crack is caused by post-welding heat treatment (e.g. stress relief heat treatment), and the elements of Cr, Mo, V, Nb, Ti in steel aggravate the tendency of reheat crack.

06 Mechanism of Reheating Crack

The reheat crack originates from the nucleation of microcracks caused by the preferential sliding of grain boundaries, and the grain boundary is weakened and the grain is strengthened in the post-weld heat treatment.

(1) The theory of weak chemical bonding at grain boundaries

The impurity elements (e.g. P, S) are brittle due to segregation at grain boundaries, while the carbon/nitride of Cr, Mo, V, Nb and other elements are strengthened by precipitation in the grain during secondary heating. The stress relaxation deformation is concentrated at grain boundaries, and the crack is caused by insufficient plasticity.

(2) Intra-crystalline strengthening theory (mode cracking theory)

The in-situ precipitation of the strengthening phase (chromium carbide, vanadium, titanium, niobium, etc.) in the dislocation region hinders the in-crystal deformation, the stress relaxation is borne by the grain boundary, and the stress concentration leads to the crack.

(3) Creep fracture theory

Crack growth along grain boundaries is accelerated by the accumulation of creep damage during the stress relaxation process.

07 Reheating crack sensitivity identification formula

The main factors affecting the reheat crack are the chemical composition of the steel (which directly affects the plasticity of the coarse grain zone) and the residual stress in the welded zone (especially the stress concentration area).

△G＝Cr＋3.3Mo＋8.1V＋10C－2

When△G<1.5, the reheat crack is not sensitive;

When 1.5<△G<2, it is generally;

When △G>2, the reheat crack is sensitive.

08 EXPERT ANALYSIS

1: During post-weld heat treatment or high-temperature service, constrained welded joints may develop 'reheat cracks' (also termed 'stress relaxation cracks') in their heat-affected zones. Historically, these cracks were first identified in austenitic stainless steels by power plant engineers. Metallurgical studies confirm their close association with intracrystalline precipitates. Essentially, these precipitates reinforce the grain interior, forcing the strain required for stress relaxation to shift to grain boundaries, thereby reducing creep ductility and causing boundary failure. This cracking mechanism remains incompletely understood, particularly regarding the microstructural and compositional factors influencing its formation.

2: The grain boundary precipitation strengthening only explains part of the cause. The grain size is too large, which will lead to the creep ductility deterioration and the grain boundary precipitation accumulation. In addition, the weld shrinkage, the coarse grain will hinder the grain boundary slip, the material thickness increase and the savage welding process will aggravate the tendency of crack.

Two types of reheating cracks:

1) The weld did not crack initially, but cracks appeared during the post-weld stress-relief heat treatment. These cracks are termed stress-relief cracking susceptibility (SR cracking susceptibility).

2) Cracks are not found after welding, but they are produced after long-term service at a certain temperature.

Characteristics of reheating crack:

1) It occurs exclusively in metal welds containing precipitation-enhancing elements (Ti, Al).

2) It only occurs in a certain temperature range, and the reheat crack is related to the reheat temperature and time, there is a sensitive temperature range for reheat cracking. For the general low alloy steel, the temperature range is about 500~700℃; for the austenitic stainless steel and some high temperature alloy steel, the temperature range is 700~900℃.

3) The crack extends along the austenite grain boundary on the weld side, and the crack direction is intergranular.

4) there must be residual stress and stress concentration in the welding area.

The formation mechanism of reheat crack is the weakening effect of impurity precipitation at grain boundary and the strengthening effect of grain interior precipitation.