HAZ Cracking Research Articles

中，高炭素低合金鋼の溶接熱影響部における低温割れの研究 ＶＩ 焼割れ的低温割れ感受性に及ぼす燐の粒界偏析およびセリウムによるその改善効果

In order to investigate the harmful effect of phosphorus on quenching crack type cold cracking in weld HAZ of medium/high carbon low alloy steels, fracture behavior and grain boundary segregation have been studied by the simulated cold cracking test and Auger electron spectroscopic analysis with vacuum induction mélted JIS SNCM447 containing low (about 0.001wt.%) and high level (about 0.030wt.%) of phosphorus and sulphur. The effect of cerium on the harmful effect of phosphorus has been also studied by the same procedures.Phosphorus enhances the intergranular embrittlement and greatly reduces fracture stress. On the other hand, sulphur does not the detrimental effect.The AES results clearly indicate that phosphorus was segregated at grain boundaries. Carbon was also found to be segregated at the grain boundaries. The phosphorus segregation to grain boundary is fairly increased by grain boundary liquation and decreased with increasing in cooling time. These tendencies are closely correlated with the fracture stress and the fraction of intergranular fracture. The decrease of phosphorus segregation in relation to increasing cooling time occurred at the temeprature region below Ms during cooling, however, this is inconsistent with general tendency of equilibrium segregation described in Fe-P binary alloy. Therefore, it was thought that some complex mechanisms seem to play a role in the temperature region.Cerium has a beneficial effect to reduce the harmful effect of phosphorus due to reducing the degree of phosphorus segregation at grain boundaries.

中，高炭素低合金鋼の溶接熱影響部における低温割れの研究 ＩＩＩ 焼割れ的低温割れに及ぼす硬さおよび化学成分の影響

For evaluation of crack susceptibility of medium, high carbon low alloy steels to quenching crack type cold cracking in HAZ during welding, the effects of hardness and chemical compositions on fracture stress during cooling have been studied by the simulated cold cracking test using commercial and laboratorial nineteen kinds of steels.The fracture stress showed a good correlation with the fraction of intergranular fracture surface. Therefore, it was understood that the fracture stress is useful to evaluate crack susceptibility of quenching crack type cold cracking. The fracture stress had a tendency to decrease with an increase in hardness. With respect to intergranular fracture, it is considered that the steels showing the hardness higher than about Hv600 are susceptible to quenching crack type cold cracking. Moreover, phosphorus is very harmful because it lowers the fracture stress due to increasing intergranular fracture surface considerably. Furthermore, the regression analyses on the fracture stress in relation to hardness and such impurity contents as P, N, Sn and O showed; (i) the fracture stress mainly depends on hardness and P content, (ii) N is harmful, too, but the effect is much less than P, (iii) Sn and O has little effect.

中，高炭素低合金鋼のＲＲＣ試験における焼割れの発生挙動 Ｉ 中，高炭素低合金鋼の溶接熱影響部における低温割れの研究

In order to investigate the weld cold cracking of medium, high carbon low alloy steels which is considered to be caused by quench cracking, the fundamental behaviors of this type of cracking were studied by means of the RRC test. Namely, HAZ cracking in JIS SNCM439 (0.4C-1.8Ni-0.8Cr-0.3Mo steel) and JIS SK5 (0.8C plain carbon steel) was studied under the conditions of restraint intensity of about 9.8 to 29.4 kN/mm·mm without preheating utilizing GTA welding with austenitic filler wire to eliminate the effect of diffusible hydrogen. Main conclusions obtained are as follows:(1) General behavior of restraint stress after welding of these steels nearly agrees with that of weldable high strength steels in which martensitic transformation reduces restraint stress.(2) The last stage which gives the maximum or saturated restraint stress after welding is not seen even in the low restraint intensity of about 10 kN/mm.mm, because the cracking develops largely before the last stage.(3) Crack initiation estimated with AE technique occurs earlier with increasing the restraint intensity. The highest temperature of crack initiation, however, was about 35- K and 393 K in SNCM439 and SK5, respectively, meaning fairly lower than their Ms temperatures.(4) The stress at the crack initiation was about 400 MPa and 270 MPa in SNCM439 and SK5, respectively irrespective of the restraint intensity.(5) Crack initiation part has an intimate correlation with liquated grain-boundary in HAZ near fusion boundary, and gives very brittle intergranular fracture surface.

Ａｌ‐５％Ｍｇ合金鋳物の電子ビーム溶接に関する一研究

In electron beam welding of Al-5%Mg alloy casting (JISH5202, Class 7 AC7A), effects of factors such as welding conditions, base metal thickness (6, 16 mm), Ti addition (Ti free, 0.12%Ti) and etc. on welding phenomena, metallurgical and mechanical properties of weld zone have been investigated by microscopic observation, thermal cycle measurement, grain size measurement, fatigue test and etc. The results obtained are as follows:(1) In welding of AC7A with 6 mm thickness, arcing phenomena are reduced by proper welding condition and weld zone with sound mechanical properties are obtained. In welding AC7A (Ti) with 6 mm thickness, arcing phenomena are prevented by beam current at 5% down of filament current and weld zone with sound mechanical properties are obtained.(2) In welding of AC7A and AC7A(Ti) with 16 mm thickness, arcing phenomena are prevented by combined conditions such as low power density of beam (ab; 1.2), preheating (130°C) by EB passes and beam current at 5% down of filament current and weld zone with sound mechanical properties are obtained.(3) Intercrystallrne cracks in HAZ are observed with AMA, 16 mm thickness welded joints. Eutectic compositions (mainly Al3Mg2) with low melting point are recognized in intercrystalline cracks in HAZ.(4) Al-5%Mg alloy casting with 0.12%Ti addition shows good weldability owing to refined grain size of base metal and HAZ.

９％Ｎｉ鋼溶接部の破壊挙動の特異性とその評価

The available data on the brittle fracture initiation properties, of 9% Ni steel weldments bear out their satisfactorily high toughness at low temperatures, but have a'little ambiguity that arises in connection with the pop-in fractures and the tendency of HAZ cracks to deviate, into the lower, strength ductile weld metal which are observed when conducting fracture toughness tests on 9% Ni steel. weldments at low temperatures. However, no basic concept by which to assess. the significance of such fracture. behavior has been established yet.With this fact in mind, and investigation was undertaken in order to arrive at a more precise interpretation of the correlation between a such fracture behavior and structural intergity of 9% Ni steel LNG storage tanks. Three-point bending COD tests using coil-spring loading equipment and wide plate tests were performed on 9% Ni steel weldments made using the austenitic Ni-based and matching ferritic fillers. In these tests the pop-in behavior was also observed to some extent at -196°C, and the tendency for elimination of pop-ins by precipitating complete failure was predominantly observed at the same level off stress intensity in COD terms. It was nevertheless shown that 9% Ni steel weld-HAZs had no doubt satisfactory toughness at the LNG temperature(-162°C).

高張力鋼溶接熱影響部における水素による割れと部分溶融 ＩＩ 割れの伝ぱ，母材部での割れおよび割れ部の組織観察

Hydrogen-assisted crack propagation and cracking in as-received HY80 steel were directly observed. Metallographic examination of cracks was performed using the etchant of picric acid with wetting agent. Direct observation of crack propagation shows that before microcracks start to grow localized yielding occurs in advance of crack tip and subsequently evolution of hydrogen occurs, and then microcracks grew into macrocracks, accompanying thus periodic process.Metallographic examinations of cracks exhibited that the incidence of cracking was predominantly high at bands and the presence of bands seemed to play very important roles in hydrogen-assisted cracking in HAZ.

高強度マルテンサイト系ステンレス鋼の溶接性

Influences of the conditions of weld thermal cycle and postweld heat treatment on the notch toughness, tensile properties and crack sensitivity in the HAZ of two high strength (80 kg/mm2 type and 100 kg/ mm2 type) martensitic stainless steels containing Ni, Mo, Nb and Cu were investigated by means of welding thermal cycle simulating method and the slit type cracking test. The results of these tests are sum-marizsed as follows.(1) In the case of 80 kg/mm2 type steel (base metal A), the impact value steeply falls when thermal cycle above 1, 300°C peak temperature is experienced, and recovery of the impact value due to the postheating is little even with postheating at 650°C for 2 hours.(2) In case of the 100 kg, /mm2 type (base metal B; precipitation hardening) steel, the impact value does not almost decrease even though peak temperature of the thermal cycle is changed within the range of Ac1 point-1, 300°C. However, postweld aged at 500°C for 4 hours after exposure to the tehrmal cycle, the impact value drops remarkably when the peak temperature of the thermal cycle exceeds 950°C.(3) In the multipass weld joint tests on two test steels which was carried out under optimum welding procedure and postweld haet treatment as shown in Table 4, satisfactory notch toughness and tensile properties were obtained.(4) In the slit type cracking test, the HAZ cracking can be perfectly prevented when the preheating temperature was higher than 200°C for base metal A and 100°C for base metal B, and holding temperature after welding was at 100°C for each kinds of the test steels and then postheated at 600°C for 10 minutes.

耐食性75Ni-15Cr-Fe合金の溶接熱影響部割れに関する研究(第4報)

A study was carried out to seek the element which did not affect the HAZ cracking of 75Ni-15 Cr-Fe alloy remarkably and improve the forgeability of it instead of Mg.The main results obtained are as follows:1. Using Ca, the range where HAZ cracking does not occur can be extended.2. Using Ca, the mechanical and corrosion properties of 75 Ni-15 Cr-Fe alloy do not become worse.3. Consequently, a weldable 75 Ni-15 Cr-Fe alloy which does not cause HAZ cracking can be made more easily using Ca.

オーステナイトステンレス鋼熱影響部における結晶粒界拳動に関する研究(第2報)

A series of experiments on stainaless steels have been conducted in order to understand the phenomenon and mechanism of weld hot cracking, especially due to grain-boundary liquation, in heat-affected zone of some steel weldments.In previous report (the lst report), some new knowledge of liquation cracking was obtained for Type 310 and Type 304 austenitic stainless steels.In this report, some experiments on Type 347 and Type 321 stabilized austenitic stainless steels were conducted with a high temperature microscope and an electron probe X-ray microanalyser to obtain a more detail understanding of grain-boundary liquation cracking in the weld heat-affected zone.The main results obtained in this study are as follows:1) In Type 347 stainless steel, the appearance of liquid phase occurred at temperature lower by about 70°C than the solidus in both cases of rapid and slow heating.This phenomenon is probably attributed to eutectic reaction between niobium carbide and r-iron.2) Grain-boundary liquation was observed in the heat-affected zone of Type 347 stainless steel. The mechanism causing grain-boundary liquation is presumed to be as follows:Grain-boundary migration accompanying rapid heating occurs and these migrating grain boundaries are pinned by niobium precipitates which remain indissolved in spite of heating at high temperature above 1300°C. Continuously, with an increasing temperature NbC-r-iron eutectic reaction occurs and then eutectic liquid produced in close vicinity of niobium precipitates penetrates into the grain boundaries which have been pinned and fixed here.3) Narrow ferrites extended in the rolling direction of the base metal were liable to promote liquation hot cracking in HAZ.4) It was presumed that Type 321 stabilized steel has less tendency to grain-boundary liquation than Type 347 stabilized steel and then has less susceptibility to hot cracking in HAZ.

耐食性75Ni-15Cr-Fe合金の溶接熱影響部割れに関する研究(第2報)

Studies were conducted to investigate the effect of chemical elements, grain size and degree of cold working on the HAZ cracking of 75 Ni-15 Cr-Fe alloy.The results obtained in this report are as follows:1. Increasing Mg content of 75 Ni-15 Cr-Fe alloy, the HAZ cracking occures remarkably.2. C, Ti and Al do not influence on the HAZ cracking within the content of specification value.3. Austenitic grain size and cold working do not effect remarkably on the HAZ cracking of 75 Ni-15 Cr-Fe alloy.

サブ・ゼロ溶接法について(第1報)

It is well known that in the case of welding high strength steel rapid cooling causes embrittlement and cold cracking in HAZ, on the other hand slow cooling causes softening in it. And the hgher the strength of steel, the more difficult it is to get enough notch toughness of weld metal as that of base metal. The purpose of this study is to weld high strength steel by austenitic steel electrode wire in order to get high weld-ing speed. And proof stress of austenitic weld metal is to be increased by refrigeration (sub-zero treatment) and aging after welding. As a rseult of the preliminary experiment by cast specimens, it was found theat the cast steel of composition of 17.5% chromium equivalent and 9% nickel equivalent will be increased in its strength and hardness by refrigeration and aging. And welded steel plate could locally be refrigerated by methylalcohol and dry ice mixture or by liquid nitrodgen.As results of sub-zero welding of HT-80 high strength steel, the following mechanical properties of weld metal were obtained after refrigration at -50°C;0.2% proof stress, 68kg/mm2tensile strength, 113kg/mm2elongation, 12%Charpy impact value, 2.8kg:-m/cm2 (at 0°C and -80°C)And the mechanical properties of weld metal refrigerated at -72°C and aged at 450°C for one hour were as follows;0.2% prooof stress, 100kg/mm2tensile strength, 126kg/mm2elongation, 15%Charpy impact value, 2.4kg-m/cm2 (at -80°C)

耐食性75Ni-15Cr-Fe合金の溶接熱影響部割れに関する研究(第3報)

The studies in this report were carried out to clarify the effect of Mg on the hot ductility of 75 Ni-15 Cr-Fe alloy and to investigate the relation between the existence of Ni2Mg and (Ni2Mg+Ni) and HAZ cracking.The results are summarized below:1. Increasing content of Mg, hot ductility of 75 Ni-15 Cr-Fe alloy becomes worse and grain boundary breaking occures remarkably.2. The existence of Ni2Mg and (Ni2Mg+Ni) is thought to be the cause of HAZ cracking in 75 Ni-15 Cr-Fe alloy.3. HAZ cracking of 75 Ni-15 Cr-Fe alloy can be prevented by means of solid solution treatment for Mg.

耐食性75Ni-15Cr-Fe合金の溶接熱影響部割れに関する研究(第1報)

Studies were conducted to investigate the effects of compositions and any other factors on the HAZcracking of 75Ni-15Cr-Fe alloy and to perform the prevention of it.In the 1th report, the characteristics of the 75Ni-15Cr-Fe alloy and the HAZ cracking of the alloy were described.The contents described in this report are as follows:1. Eutectic structure (Ni2Mg+Ni), which is substituted partially by Cr and Fe in a portion of Ni, crystalizes in 75Ni-15Cr-Fe alloy containing Mg2. Grain boundary weld cracking is occured in the HAZ of 75Ni-15Cr-Fe alloy3. HAZ cracking of 75Ni-15Cr-Fe alloy is affected rather by the chemical composition than by welding method, type of welding joint and thickness of plate

オーステナイトステンレス鋼熱影響部における結晶粒界挙動に関する研究(第1報)

A series of experiments on stainaless steels have been conducted in order to understand the phenomenon and mechanism of weld hot cracking, especially due to grain-boundary liquation, in heat-affected zone of some steel weldments.In previous report (the lst report), some new knowledge of liquation cracking was obtained for Type 310 and Type 304 austenitic stainless steels.In this report, some experiments on Type 347 and Type 321 stabilized austenitic stainless steels were conducted with a high temperature microscope and an electron probe X-ray microanalyser to obtain a more detail understanding of grain-boundary liquation cracking in the weld heat-affected zone.The main results obtained in this study are as follows:1) In Type 347 stainless steel, the appearance of liquid phase occurred at temperature lower by about 70°C than the solidus in both cases of rapid and slow heating.This phenomenon is probably attributed to eutectic reaction between niobium carbide and r-iron.2) Grain-boundary liquation was observed in the heat-affected zone of Type 347 stainless steel. The mechanism causing grain-boundary liquation is presumed to be as follows:Grain-boundary migration accompanying rapid heating occurs and these migrating grain boundaries are pinned by niobium precipitates which remain indissolved in spite of heating at high temperature above 1300°C. Continuously, with an increasing temperature NbC-r-iron eutectic reaction occurs and then eutectic liquid produced in close vicinity of niobium precipitates penetrates into the grain boundaries which have been pinned and fixed here.3) Narrow ferrites extended in the rolling direction of the base metal were liable to promote liquation hot cracking in HAZ.4) It was presumed that Type 321 stabilized steel has less tendency to grain-boundary liquation than Type 347 stabilized steel and then has less susceptibility to hot cracking in HAZ.

ルート割れと高張力鋼溶接部組織の関係について

Effects of microstrure on root cracking of high strength steel welds were discussed in this report, mainly with the use of SH-CCT diagrams of test steels. Critical hardness values for root cracking were also discussed.The main conclusions obtained in this study are as follows :(1) A safety cooling process to prevent root cracking in HAZ should be chosen so that the ferrite and intermediate structure may appear more than about 40% for HT 60, intermediate structure more than about 25% for HT 70, and more than about 10% for HT 80 steels, respectively, when a joint of severe restraint is welded with a low hydrogen type electrode.(2) For a weld metal, it is necessary to prevent root cracking in a severe restraint joint that the ferrite and intermediate structure precipitate more than in HAZ.(3) A root crack in HAZ or weld metal is a mixed fracture of transcrystalline and intercrystalline types.(4) There was found no relationship between root cracking tendencies and Mf temperatures.(5) Retained austenite in HAZ was decomposed with the lapse of time above 250°C, but did not affect the root cracking phenomena.(6) Critical hardness, above which a root crack will surely occur in a joint of severe restraint, were found to be as follows :270 VHN (1 kg) for HAZ of mild steel weldment365 VHN (1 kg) for HAZ of HT 50 weldment400 VHN (1 kg) for HAZ of HT 60 weldment405 VHN (1 kg) for HAZ of HT 70 weldment415 VHN (1 kg) for HAZ of HT 80 weldment290 VHN (1 kg) for weld metal of HT 50 and HT 60 weldments330 VHN (1 kg) for weld metal of HT 70 and HT 80 weldments