Reheat Cracking Research Articles

Round robin of trace elements

A round robin of measurement of trace elements P, As, Sb, Sn, Pb, and Bi in 2 1/4 Cr–1 Mo–V steel has been conducted in IIW Commission II. Samples of three heats were distributed among seventeen laboratories. Results were used to calculate the Bruscato and Watanabe factors for temper embrittlement resistance, which factors are used commercially to accept or reject matching steel filler metals for elevated temperature service. Likewise, results were used to calculate a reheat cracking factor proposed for acceptance or rejection of heats of matching steel filler metals to resist reheat cracking during extended postweld heat treatment. The results obtained show quite acceptable reproducibility of measurement of elements comprising the Bruscato and Watanabe factors in which statistically significant differences among the heats could be found. But the results obtained show very poor reproducibility of measurement of Pb and Bi so that statistically significant differences in the calculated reheat cracking composition factor could not be found among the heats unless the analytical method was restricted to ICP-MS.

Study of creep cavitation in stainless steel weldment

A study of creep cavities near reheat cracking in AISI Type 316H austenitic stainless steel headers, removed from prolonged high temperature operation in nuclear power plants, is reported. It is shown how application of scanning electron microscopy (SEM), cryogenic fractography and small angle neutron scattering (SANS) can be applied, in a complementary way, to observe and quantify creep cavitation. Creep cavities in the vicinity of the crack are found to be mainly surrounding inter-granular carbides. Trends in the size and area fraction of creep cavities relative to the crack path are quantified using optimised metallography. The SANS technique is found to be a very suitable method of quantifying creep cavitation within the size range up to 600 nm averaged over a larger gauge volume. It is shown that the cavity size distribution peaks in the region 100–300 nm, and this correlates closely with the quantitative SEM observations.

レーザ顕微鏡を用いたその場観察・計測法による再熱割れ感受性の定量評価

レーザ顕微鏡を用いたその場観察・計測法による再熱割れ感受性の定量評価

Evaluation of weld metal hot cracking susceptibility in superaustenitic stainless steel

Solidification cracking susceptibilities of two types of superaustenitic stainless steel, 254SMO and SR50A, were evaluated by transverse Varestraint tests. The susceptibilities were compared with those of conventional austenitic stainless steel 316L, and factors influencing the difference of susceptibility were discussed. The comparison showed that 254SMO and SR50A are more sensitive to solidification cracking than 316L. In the transverse Varestraint tests, both total and maximum crack lengths are longer in the superaustenitic stainless steel. Because of the longer maximum crack length, the superaustenitic stainless steel also has a wider brittleness temperature range of cracking than 316L: about 178 °C for the superaustenitic stainless steel and 43 °C for 316L. It is believed that straight subgrain boundaries owing to the cellular dendritic solidification and segregations of sulfur and phosphorus in the subgrain boundaries of superaustenitic stainless steel make it more sensitive to solidification cracking. In addition to the solidification cracking, reheat cracking is also observed within the previous weld bead in the superaustenitic stainless steel because of fully austenitic solidification with significant segregations. This suggests that caution should be given to the occurrence of reheat cracking when superaustenitic stainless steel is multi pass welded.

Failure Analysis and Welding Repair for Reheater Tube Crack of 12Cr1MoV Steel

Through methods, such as microscope analysis, chemical analysis, hardness analysis, metallography examination etc, the causes resulting in reheater tube cracking of 300MW station boiler have been analyzed. The result indicates this reheater tube cracking is reheat crack. The higher sensitive temperature in operating, excessive welding residual stress are the primarily inducement of cracking. In welding repair, the measures , such as replacing the material ,lower welding heat input, preheat treatment and so on, were taken. The welding repair succeeded greatly.

Failure Analysis on Heating Surface Tube Crackof High Temperature Reheater in Station Boiler

Through methods, such as microscope analysis, chemical analysis, metallography examination and scanning electron microscope etc, the causes resulting in high temperature reheater tube cracking of station boiler have been analyzed. The result indicates the crack is reheat crack, and structure stress, higher hardness and excessive welding residual stress are the primary inducement of tube joint crack.

Study on grain boundary character and strain distribution of intergranular cracking in the CGHAZ of T23 steel

Intergranular reheat cracking in the coarse-grained heat-affected zone of T23 steel was produced by strain to fracture tests on a Gleeble 3500 thermal–mechanical simulator. Then the grain boundary character, as well as the strain distribution after reheat crack propagation, was studied by electron backscatter diffraction technique. The results showed that incoherent Σ3 boundaries were seldom found on the prior austenite grain boundaries. Therefore, only the type of random high-angle boundaries played a crucial role in the intergranular cracking. Microstructurally cavities and small cracks were preferentially initiated from high-angle grain boundaries. Low-angle grain boundaries and high-angle ones with misorientation angles less than 15° were more resistant to the cracking. More importantly, the fraction of high-angle grain boundaries increased with the plastic strain induced by both temperature gradient and stress in the coarse-grained heat-affected zone, which contributed to the crack initiation and propagation. Furthermore, the strain distributions in the vicinity of cavities and cracks revealed the accommodation processes of plastic deformation during stress relaxation. It also reflected the strength differences between grain interior and grain boundary at different heat-treated temperatures, which had a large influence on the cracking mechanism.

Reheat cracking in Ti2AlNb alloy resistance spot weldments

Serious reheat cracking susceptibility was found in Ti2AlNb alloy resistance spot weldments and the cracking mechanism was investigated in this work by means of hot ductility test and microstructure analysis. The results indicated that the precipitation of hardening phase (O phase) together with the residual stress have led to cracking during post-weld heat treatment in Ti2AlNb alloy resistance spot weldments. To prevent the reheat cracking, rapid heating schedule or mechanical stress relieving method was proposed and verified.

Analysis on Welding Crack of Subcritical Power Station Boiler Platen Superheater Header

Welding crack analysis was carried out on tee joint crack of boiler platen superheater header (12Cr1MoV) of subcritical power station applying finite element analysis,microanalysis, mechanical properties test, chemical composition analysis, metallography examination and SEM observation etc. Results indicated that the improper structure, the original welding defects, higher welding heat input, excessive welding residual stress and improper heat treatment led to cold crack and reheat crack ,then induced to the leakage failure while operating.the welding crack originated at the original defects, connected with reheat cracks during propagation and led to leakage finally.

Research on Welding Repair for Tee Joint Crack of Subcritical Power Station Boiler Platen Superheater Header

Through methods, such as microscope analysis, metallography examination, the causes resulting in tee joint crack of subcritical power station boiler platen superheater header have been analyzed. The result indicates the tee joint crack originated in the original defects, expanded to connect with reheat cracking and leaked finally. In repair crack, the measures of reducing residual stress, such as lower welding heat input, reducing thickness of welding layer, hammer blowing, assigning reasonably heat treatment power ,lengthening holding time and so on, were taken. The welding repair succeeded greatly.

Research on Welding Repair for Tee Joint Crack of Final Superheater Header of 300MW Station Boiler

Through methods, such as microscope analysis, metallographic examination, the crack located nearby the fusion line between tee joint and outlet steam pipe of subcritical power station boiler final superheater header is reheat crack (the main crack occurred in the heat affected zone and extended along the fusion line and grain boundary in HAZ. There are many small cracks paralleling with main crack). The base metal material (12Cr1MoV) and excessive welding residual stress are the primary inducement of crack. In repair crack, the measures of reducing residual stress, such as lower welding heat input, reducing thickness of welding layer, hammer blowing, assigning reasonably heat treatment power ,lengthening holding time and so on, were taken. The welding repair succeeded greatly.

Effect on the chemical activity of flux on the properties of deposited metal in electroslag welding and remelting

The effect of the chemical activity of flux on the properties of deposited metal of Mn–Ni–Mo–V steels, produced by electroslag welding and electroslag remelting, is discussed. It is shown that an increase in the chemical activity of flux by increasing its silica content to 16–24% (FTs-21 flux) in the remelted metal increases the fraction of easily deformed non-metallic oxide inclusions and the number of zones with reduced hardness in the structure of the metal. This increases the elasticity properties and strain capacity of the metal in the process of relaxation of welding stresses and in the formation of the required mechanical properties of the welded joints after final heat treatment. Also, the risk of formation of reheat cracks in the electroslag welded joints is reduced.

Failure Analysis on Tee Joint Crack of Subcritical Power Station Boiler Platen Superheater Header

Failure cause analysis was carried out on tee joint crack of boiler platen superheater header (12Cr1MoV) of subcritical power station applying microanalysis, mechanical properties test, chemical composition analysis, metallography examination and SEM observation etc. Results indicated that the tee joint crack originated at the original defects, connected with reheat cracks during propagation and led to leakage finally. The improper structure, the original welding defects, overhigh welding heat input, and excessive welding residual stress resulted in the leakage failure.

Research on Welding Repair for Reheating Hot Pipe Elbow of 325MW Station Boiler

Through methods, such as microscope analysis, metallographic examination, the crack located nearby the fusion line between reheating hot pipe elbow and blocking valve is reheat crack. The base metal material and excessive welding residual stress are the primary inducement of crack. In replacing pipe elbow, the measures of reducing residual stress, such as hammer blowing ,lower welding heat input, post welding heat treatment and so on, were taken. The welding repair succeeded greatly.

Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened by Cu-rich precipitates and M2C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms. Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic (face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high Ni content of the BA-160 alloy.

Constitutive equations that describe creep stress relaxation for 316H stainless steel at 550°C

The prediction of the stress relaxation behaviour of welding induced residual stresses in thick section 316H austenitic stainless steel welded component provides an input for quantifying reheat crack initiation observed in the heat affected zone. The cracks occur after service at a temperature range from 490 to 520°C. The present work reviews some of the widely applied stress relaxation models. The relative strengths and weaknesses of these existing models are discussed. An improved constitutive equation derived from a forward uniaxial creep deformation law is proposed. The relative importance of the parameters selected in the new constitutive model, when compared with experimental data, is discussed. The importance of a better understanding of the role of the internal stress and its measurement is highlighted.

Study on the Weldability of ZG30Cr06A Needling Material

ZG30Cr06A as a newly developed material is made into needling of mining machinery’s hydraulic support. Its tensile strength is up to 800MPa. In the paper, the weldability of ZG30Cr06A was thoroughly studied. The sensitivity to cold crack of ZG30Cr06A was tested by means of maximum hardness of HAZ and Y-slit type cracking test. And the variation of microstructure and properties of weld joints under different welding energy conditions was studied. The results show that ZG30Cr06A steel has a certain cold crack and hot crack tendency and a little reheat crack sensitivity in welding. Embrittlement and softening often appear in the HAZ. When the workspaces of 30mm were welded at room temperature, the maximum hardness of HAZ was up to 389HV. And the joints have a tendency to quenched hardening and bigger sensitive to cold crack. The welded joints would not produce cold crack when the preheat temperature is up to 120°C in welding. Because there is more influence of welding energy input on the microstructure and properties of joint, so in order to avoid the appearance of embrittlement and softening in HAZ and ensure weld quality, the welding energy input of ZG30Cr06A steel whose thickness is 30mm should be controlled less than 40 kJ/cm during the welding.

Failure Analysis on Tee Joint Crack of Subcritical Power Station Boiler Final Superheater Header

Through methods, such as microscope analysis, mechanical analysis, chemical analysis, metallographic examination, the causes resulting in tee joint crack of subcritical power station boiler final superheater header (12Cr1MoV) have been analyzed. The result indicates the tee joint crack is reheat crack, and irrational structure and excessive welding residual stress are the primary inducement of tube joint crack.

Constitutive equations that describe creep stress relaxation for 316H stainless steel at 550°C

The prediction of the stress relaxation behaviour of welding induced residual stresses in thick section 316H austenitic stainless steel welded component provides an input for quantifying reheat crack initiation observed in the heat affected zone. The cracks occur after service at a temperature range from 490 to 520°C. The present work reviews some of the widely applied stress relaxation models. The relative strengths and weaknesses of these existing models are discussed. An improved constitutive equation derived from a forward uniaxial creep deformation law is proposed. The relative importance of the parameters selected in the new constitutive model, when compared with experimental data, is discussed. The importance of a better understanding of the role of the internal stress and its measurement is highlighted.

Microstructural sensitivity of 316H austenitic stainless steel: Residual stress relaxation and grain boundary fracture

The present work considers the role of thermo-mechanical history on the generation and relaxation of residual stresses, typical of those encountered in Type 316H austenitic stainless steel thick section weldments. A series of thermo-mechanical pre-treatments have been developed and applied to simulate the critical microstructures observed within the heat affected zone of the thick section parent material. The through thickness distributions of the residual macro-stresses in cylindrical specimens have been measured by neutron diffraction and then the rates of the relaxation are shown to be a function of microstructure. The susceptibility to intergranular brittle fracture at a temperature of −196 °C is shown to be a function of M 23C 6 carbide precipitates and phosphorous segregation at the grain boundaries. Finally, the link of the present study to the understanding of the reheat cracking is briefly discussed.