HT-9 Steel Research Articles

ＨＴ８０鋼溶接部における遅れ割れとその破壊力学的評価 Ｉ 多層盛継手における遅れ割れ特性とＡＥ

Delayed crack initiation and propagation behavior from a notch of poor penetration in a weldment of HT80 steel of 31 mm thickness has been investigated, to apply a fracture mechanical technique to a evaluation of the susceptibility to delayed cracking. Three points bend specimen of 70 mm width with weld defect of poor penetration were welded, and the tip of the notch was located on a fusion line in modified-Vee joints. Crack opening displacement and Acoustic Emission signals given off have been recorded as a function of time. Results can be summarized as follows:(1) Fracture mode of the presure vessel at proof loading, that is the transition behavior from delayed cracking to unstable cleavage fracture, can be easily reappeared by the laboratory test proposed in this report.(2) The susceptibility to delayed cracking of the weldment of HT80 steel can be successfully evaluated as a function of stress intensity factor or COD.(3) Specimen actively emits Acoustic Emission accompanying delayed crack growth.

Counter Method as X-Ray Microbeam Diffraction Technique and Its Application to Stress Concentration Problems

In the present paper, the applicability of the counter method for stress measurement as one of the X-ray microbeam diffraction techniques is discussed. The stress can be measured at a local area of 330μ in diameter for Cr powder and 450μ for HT60 steel by using the crystal oscillation X-ray microbeam diffraction technique. The stress measured by this method agrees with the applied stress, and therefore, this method is suitable for both residual and applied stresses.As an example of the application of the method, internally notched plates were stretched and the stress distributions in the vicinity of notch tips were measured. The stress distributions obtained experimentally at the notch tips were similar to those calculated by the finite element method in the elastic regime. In the plastic regime, however, there was a slight discrepancy between them because of the influence of the yielding conditions of specimens.It is concluded that the counter method is a very effective mean for the studies of stress concentration problems and its application to the field of strength of materials is promissing.

Influence of stress ratio on the threshold level for fatigue crack propagation in high strength steels

Centrally cracked specimens of JIS SM58Q and HT80 steels were fatigued. The fatigue crack growth rates, da/ dn, and the stress intensity threshold levels, ΔK th were measured over the range of stress ratio, R, from −1 to 0.8 by the use of an automatic method of continuously decreasing stress intensity factor with crack extension. The measured ΔK th was well represented as | ΔK th /2| R =(1− R) γ| ΔK th /2| R=0 ; and the propagation rate, as d a/d n = A(1− R) γm [( ΔK/2) m − {(1 − R) γ | ΔK th /2| R=0} m ] for −1≦ R≦0.33 or d a/d n = A(1 − 0.33) − γm [( ΔK/2) m {(1 − R) γ | ΔK th /2| R=0} m ] for 0.33 < ≦ 0.8.

高張力鋼のサブマージアーク溶接における溶接割れの研究 Ｉ 横割れについて

It has been well known that there often occur some kinds of hydrogen-induced cracking, such as transverse cracking and root cracking, in the submerged-arc welding of HT-80 steels. There have been many studies on them individually, but would be very few on all-round quantitative conditions to prevent them.Therefore, as the first stage, the prevention of the transverse cracking has been dealt with here.The conclusions obtained are as follows;1) Experimental equations are proposed for preventing the transverse cracking as functions of Ceq of electrode wire or POM of weld metal, and diffusible hydrogen content and critical cooling time of weldment.2) The residual stress may be ignored in the conditions for the prevention of the transverse cracking.3) A new method for the transverse cracking test has been developed to conduct the experiment more easily.

高張力鋼のサブマージアーク溶接における溶接割れの研究 ＩＩ ルート割れについて

In the previous study, the welding condition for preventing the transverse weld cracking in submerged-arc welding of HT-80 were determined.In the present study, y-groove cracking tests were employed for determining the root cracking condition.The root cracking characteristics depend on the PCM of base metal or weld metal, whichever is higher, diffusible hydrogen content of weld metal and cooling time of weldment.The critical conditions to prevent the transverse cracking are severer than the ones to prevent the root cracking. It is concluded, therefore, that an adoption of the welding conditions to prevent the transverse cracking generally guarantees both the root and the transverse crack-free weldments in submergedarc welding of HT-80 steels.

The retardation phenomenon of fatigue crack growth in HT80 steel

The fatigue crack growth behavior resulting from single and multiple applications of overload was investigated for HT80 steel. A peak load was found to cause retardation of the crack growth rate, which becomes stronger with increasing the peak/baseline stress ratio or with decreasing baseline stress intensity. Multiple overloads resulted in additional retardation. These experimental data were explained according to a new model based on crack closure conception, being correlated with the residual plastic zone size ahead of crack tip induced by the peak overload(s). The proposed formulation of retardation was expressed simply as a function of peak/baseline stress ratio, r, and two material parameters, m and β. m the exponent parameter in Paris equation and β is the ratio of crack distance at the maximum retardation to the residual plastic zone size. The retardation was predicted to increase with increasing r and m and with decreasing β. It was suggested that the parameter, β, reflects the change in the morphology of crack tip resulted from the application of overload, which determines the shape of the curve for retarded crack growth rate vs crack distance.

鋼材溶接継手の低温加熱急冷疲れ強さ改善処理のじん性におよぼす影響

It was already reported that quenchings from a temperature under A1 point improved the fatigue strength of steel welded joints, and the reasons of this fatigue strength improvement was attributed to quench aging effect and compressive residual stress in surface layer.On the other hand, aging of steel has a liklihood to make a embrittlement of steel. This experiment was carried out to clarify the effect of this fatigue strength improving heat treatment (low temperature quenching) on the toughness of welded joint.Each welded joint of rimmed steel, 50 kg/mm2 and 60 kg/mm2 steel was heated by hot dip galvanizing, at 500°C and 700°C in furnace, followed by quenching into about 20°C water. Toughness of toe of welded joint was investigated by i) Standard V notch Charpy Impact Test ii) COD Test and iii) ESSO Test.The following results were obtained, 1) The results of Charpy impact test and COD test showed that toughness of SS41, SM50 and HT60 steel welded joints did not decreased remarkably compared with as welded joint by a low temperature quenching.2) The ESSO test result showed that crack arrest temperature of SM50 steel welded joint slightly increased by low temperature quenching, but crack arrest temperature of SS41 and HT60 steel welded joints were nearly same compared with as welded joints.

Acoustic Emission during Fracture Toughness Testing of HT80 Steel

Acoustic Emission during Fracture Toughness Testing of HT80 Steel

Studies on the Fatigue Crack Initiation and Propagation : Report 1, Effects of Crack Length, Stress Range and Minimum Stress on Fatigue Crack Propagation

The fatigue crack propagation behaviors were investigated on HT80 steel plates under net section stress control in which stress range and minimum stresses were widely varied. The fatigue crack growth rates were proportional to the effective crack length 2ae which was an actual crack length 2a modified by the finite width effect of the specimen. When minimum stress δmin was in compression side, the growth rate depended on maximum stress δmax and in tension side, it depended on stress range Δδn. As the result, the effects of crack length, stress range and minimum stress on the fatigue crack propagation can unifyingly be shown such that the crack growth rate is proportional to the cyclic plastic zone size at a crack tip[numerical formula]. Where, δy is yield strength of material and δn is δmax when δmin≤0 and is Δδn When δmin≥0

厚板HT-80鋼のサブマージアーク溶接の研究(第2報)

厚板HT-80鋼のサブマージアーク溶接の研究(第2報)

厚板HT-80鋼のサブマージアーク溶接の研究(第3報)

厚板HT-80鋼のサブマージアーク溶接の研究(第3報)

高張力鋼熔接ボンド脆性破壊の防止に関する研究(第1報)

A study was made on prevention of brittle fracture in a high heat input welded joint for 80kg/mm2 high tensile steel and a metallurgical investigation into the improvement of cleavage fracture strength of weld bond was carried out.In this report the fracture characteristics of conventional HT80 steels' welded joints welded with 80 KJ/cm heat input are clarified. They fracture at weld bond in a brittle manner at 15-20°C.And the clue to prevention of brittle fracture is discussed according to fracture analysis diagram.It is stressed that the solution to this problem lies in the fact that the compound microstructure of low carbon bainite and low .carbon martensite has superior notch toughness and higher yeild stress.Namely, the higher the notch toughness and the higher the yield stress are in weld bond, the tendency of weld bond brittle fracture becomes the lis less conspicuous.

HT80母材の応力緩和特性

In order to make clear the effect of stress relief heat treatment on the reduction of residual stress in base metal, high temperature tensile test, creep test and tensile relaxation test, both anisothermal and isothermal, were carried out using two kinds of HT80 steels. Relaxation property was discussed on the basis of creep phenomena and a method of forecasting stress drop during S.R. treatment was studied. The findings are summarized as follows:1. In the anisothermal stress relaxation test, stress drop during heating was larger than that predicted from high temperature tensile properties. This fact points to the occurrence of creep strain during heating. this creep strain became larger with increasing initial stress and with temperature rise.2. In the isothermal stress relaxation test, stress drop occurred mainly in the initial 1 hour from beginning. The isothermal stress relaxation curve could be calculated from creep test data on the application of strain hardening concept.3. A higher heating rate during anisothermal stress relaxation test led to an increase in residual stress immediatelly after heating, but this effect became almost negligible as holding continued more than 1 hour and then the stress decreased to a certain value regardless of heating rate.

鋼のおくれ破壊におよぼす鋼中の水素拡散の影響(第1報)

Hydrogen embrittlement of steel has been treated as one of the phenomena of the delayed fracture. In this case, the applied stress and the behavior of hydrogen in steel have a great importance.In the previous studies, the behavior of hydrogen in steel has been examined by the experiments of hydrogen permeability through the steel membrane and hydrogen occuludability in steel at room temperature. On the other hand, authors have investigated the relation between the applied stress and the fracture time of hydrogenated steel.In the present paper, an effect of, the behavior of hydrogen on the phenomena of the delayed fracture was examined by using Constant Load Rupture testing. The tensile specimens were hydrogenated by cathodic method under constant load. The relation among the applied stress, the diffusion distance and the fracture time was examined.The results obtained are as follows;The square of hydrogen diffusion distance and the fracture time of specimen are linearly a related. Therefore, diffusion of hydrogen in steel is thought to depend upon Fick's law, as aturhors described previ-ously, and the diffusion coefficient of hydrogen in HT-80 steel is in the order of 10-6-10-7 Cm2/sec in this experimental condition.Moreover, in the case of C.L.R. testing, if the specimens are applied with a stress, varying the diffusion distance of hydrogen, the concentration of hydrogen in the fracture part of specimens would be always constant and the difference of fracture time would depend upon the time when hydrogen is segregated to the same concentration through various distances of diffusion.

鋼の定荷重下における水素ぜい化に関する研究(第3報)

In the previous paper, authors reported on the hydrogen embrittlement of several kinds of commercial steels in the constant load rupture test.In this paper, authors report on the effect of U- and V-notches on the hydrogen embrittlement of steels; mild steel, HT-60 steel, HT-80 steel and the weld joint of HT-60 steel, in the C.L.R. test. Hydrogen was charged into steel specimen cathodically under given constant load.The results obtained are summarized as follows;(1) The sensitivity of hydrogen embrittlement of steels is affected by the shape of notches, and steels having a severe notch are subject to more remarkable embrittlement than ones having a relatively dull notch.(2) The fracture of the specimens with the notched part insulated occurs after longer loading period than the case of notched specimens directly charged, under the same stress.(3) From the results on the weld joint specimens, it was known that hydrogen exerts a serious influence upon the weld part and bonding part.

溶接継手のルート割れにおよぼす外的拘束の影響

Control of weld cracking is one of the most important problems for welding engineers. Weld cracking is influenced by welding stress during cooling as well as properties of weld metal and heat affected zone. The problems of weld cracking, therefore, should be explored not only from metallurgical strandpoint but also from standpoint of welding stress. In this report research was focused on the influence of restraint on weld cracking.A new test apparatus as shown in Figs. 3 and 4 was designed for evaluation of sensitivity for root cracking of steel welds. With this apparatus, Rigid Restraint Weld Cracking test (RRC test) as shown in Fig. 1 was developed, it is such that the length of a specimen is kept constant during cooling. The RRC test method is similar to the TRC test developed by Dr. Suzuki and his collaborators for the researches on root cracking of high strength steel welds. However the authors' test method differs from the TRC test as follows : In the TRC test a constant load was applied at a few minutes or immediately after welding because emphasis was laid on only delayed root cracking of high strength steel welds. The authors' test method is very significant, on the other hand, for the two reasons which (1) it gives gradual increase of reaction load by keeping the length of a specimen always constant during welding and cooling and also (2) it permits a wide degree for adjustability in the restraint intensity by changing the gauge length of restraining (l), (See Fig. 1). This may be much similar to the actual behavior of restrained welds and will be reasonable as the test method for weld cracking under restraint during cooling as well as after cooled to room temperature.Experiments were carried out on the weldments of a mild steel and high strength steels of 60 and 80 kg/mm2 tensile strength level.The results obtained are as follows.(1) In the RRC test, shinkage rate and magnitude of reaction stress were widely changed with changing the gauge length of restraining, and weld crack occurs in wide range of temperature. Reaction load increases gradully with cooling, and higher intensity of restraint usually favours crack initiation or decreases the time required for cracking.(2) In mild steel weldments, weld crack occurs only during cooling and no crack initiation is observed after cooled to room temperature.(3) In high strength steel weldments, on the other hand, crack initiation was observed not only during cooling but also after cooled to room temperature.(4) This delayed cracking was observed even at lower reaction stress level. Minimum reaction stress level for the delayed cracking is 33 kg/mm2 for HT-80 steel and 60 kg/mm2 for HT-60 steel. The incubation period was measured 1-10 hours for HT-80 steel and 20 hours for HT-60 steel after welding.(5) Critical gauge length (lcr) of restraing, over which no crack initiation was observed, was 300-370 mm for mild steel, 450-500 mm for HT-60 steel and 550-600 mm for HT-80 steel.