Effect of hydrogen,restraint and welding conditions on weld metal cold cracking of HSLA steels in the IRC test

The effect of heat input and hydrogen partial pressure of the TIG process without filler wire and of restraining conditions in the IRC test on hydrogen induced cracking have been investigated for weld metals of StE 380, 15 NiCrMo 10 6 (HY 80) and 12 Ni 19. The weld metals are fused from the roots of the plate materials exclusively. The continuously measured combined nominal stresses transverse to the welds are strongly dependent on transformation stress release prior to cold cracking of the respective weld metals. Higher restraints are increasing nominal stresses. Critical heat inputs for prevention of hydrogen weld metal cracking are increased with restraint, hydrogen potentials and carbon contents of the weld metal, 12 Ni 19 revealing no fracture.     

Concept and procedure of the IRC test for assessing hydrogen assisted weld cracking

The self restraining Instrumented Restraint Cracking (IRC)-test enables the determination of weld joint reaction forces and -moments responsible for hydrogen assisted cracking. With help of continuous measurements during cooling of the weld, the effects of crack inducing parameters can be detected in detail. The IRC-test can be applied to procedure development and procedure qualification welding, making use of current fabrication practice at restraint intensities of the to-be-fabricated structure. It is easy to carry out and the results are quickly at hand.     

Effect of oxygen on hydrogen cracking in high-strength weld metal

The effect of oxygen content on the susceptibility of high-strength weld metal to hydrogen cracking is examined. Increasing oxygen content had a detrimental effect on the cracking susceptibility of weld metal containing a dψusible hydrogen content of 4.7 ppm. In weld metal containing a much lower dψusible hydrogen content (0.87 ppm), increasing weld metal oxygen content had no detrimental effect on hydrogen cracking susceptibility. These results are explained by a model which proposes that hydrogen cracking occurs when a critical oxide inclusion density promotes intergranular fracture at prior austenite grain boundaries and when a critical level of hydrogen is present in the weld metal. For the same level of hydrogen (moisture) contamination, high-strength weld metals containing oxygen contents greater than 200 ppm will be much more susceptible to hydrogen cracking than deposits made using inert gas-shielded or vacuum-operated welding processes. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto     

Effect of oxygen on hydrogen cracking in high-strength weld metal

The effect of oxygen content on the susceptibility of high-strength weld metal to hydrogen cracking is examined. Increasing oxygen content had a detrimental effect on the cracking susceptibility of weld metal containing a dψusible hydrogen content of 4.7 ppm. In weld metal containing a much lower dψusible hydrogen content (0.87 ppm), increasing weld metal oxygen content had no detrimental effect on hydrogen cracking susceptibility. These results are explained by a model which proposes that hydrogen cracking occurs when a critical oxide inclusion density promotes intergranular fracture at prior austenite grain boundaries and when a critical level of hydrogen is present in the weld metal. For the same level of hydrogen (moisture) contamination, high-strength weld metals containing oxygen contents greater than 200 ppm will be much more susceptible to hydrogen cracking than deposits made using inert gas-shielded or vacuum-operated welding processes. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto     

Calculation of weld metal composition change in high-power conduction mode carbon dioxide laser-welded stainless steels

The use of high-power density laser beam for welding of many important alloys often leads to appreciable changes in the composition and properties of the weld metal. The main difficulties in the estimation of laser-induced vaporization rates and the resulting composition changes are the determination of the vapor condensation rates and the incorporation of the effect of the welding plasma in suppressing vaporization rates. In this article, a model is presented to predict the weld metal composition change during laser welding. The velocity and temperature fields in the weld pool are simulated through numerical solution of the Navier-Stokes equation and the equation of conservation of energy. The computed temperature fields are coupled with ve-locity distribution functions of the vapor molecules and the equations of conservation of mass, momentum, and the translational kinetic energy in the gas phase for the calculation of the evap-oration and the condensation rates. Results of carefully controlled physical modeling experi-ments are utilized to include the effect of plasma on the metal vaporization rate. The predicted area of cross section and the rates of vaporization are then used to compute the resulting com-position change. The calculated vaporization rates and the weld metal composition change for the welding of high-manganese 201 stainless steels are found to be in fair agreement with the corresponding experimental results.     

Influence of bulk and grain boundary phosphorus content on hydrogen induced cracking in low strength steels

The influence of bulk and grain boundary phosphorus content in carbon-manganese-steels was studied by constant extension rate test method in 1 m H₂SO₄ solution under cathodic polarisation. The hydrogen activity and uptake in the samples was measured using the electrochemical permeation technique. The susceptibility towards hydrogen induced cracking (HIC) increased with phosphorus content, the steel containing lower manganese was found to be more susceptible to HIC. The mode of cracking was mostly transgranular. The effect of phosphorus is related to the bulk content and not to the grain boundary concentration. The susceptibility towards HIC is directly related to the hydrogen uptake which increases with the phosphorus content and decreases with the manganese content of the steels.     

Effect of heat input and preheating on hydrogen assisted weld joint cracking of a 0.13% C, 1.5% Mn, 0.032% Nb high strength steel of 50 mm plate thickness in the IRC test

The effects of preheating and heat input on hydrogen assisted weld joint cracking are investigated at a restraint intensity of 32 kN mm^?2 of a 0.13% C, 1.5% Mn, 0.032% Nb high strength steel of 50 mm thickness in the IRC test, using a high hydrogen experimental electrode of 530 N mm^?2 yield strength. For a heat input ranging from 0.6 to 1.05 kJ mm^?1 a critical preheating temperature of 140°C for almost complete crack prevention, for a range from 1.5 to 2.05 kJ mm^?1, 120°C were found respectively. Nominal stresses at the ends of the 70-80 mm long welds at the start of extensive cracking increase with heat input and preheat, the crack propagating from the HAZ into the weld metal quickly. Under conditions without or close to cracking, however, final stresses after 18 h are reduced with heat inputs. Consequently, crack critical combinations of preheating and heat input are linked to stresses decreasing with heat input but increasing with preheat. From the established IRC-test diagram required combinations of local preheat and heat input for either avoiding hydrogen cracking or overstressing of the weld metal can be determined. Currently used cracking prediction procedures do not consider the effect of heat input and preheat on stress sufficiently and, therefore, may provide unsafe conclusions.     

Relevance of ROT control for hot rolled low carbon steels

The need for precision in controlling coiling temperature and cooling profiles on the run out table for hot rolled low carbon steel strips has been investigated. It is claimed in literature that a high degree of automation and control of the run out table is required to control the yield strength variation to within 20 MPa. However, calculations based on models available in literature predict that the variation in strip temperature encountered in a typical run out table does not affect the mechanical property significantly. This has been shown to be true even from experimental data available in literature. Hence even a coarse control over the run out table is adequate to achieve the desired yield strength. A detailed report concentrating on the influence of coiling temperature on the mechanical properties of HSLA steels is under preparation.     

Studies of the orientations of fracture surfaces produced in austenitic stainless steels by stress-corrosion cracking and hydrogen embrittlement

Orientation studies have been made on several different austenitic stainless steels, using photogrammetric and electron channeling techniques. The fracture facets produced by SCC in boiling aqueous MgCl₂ (155 °C) were large and relatively flat in the case of type 310 steels, and the fracture plane was found to be at or near {100}. The transgranular stress-corrosion fractures in type 304 steels were more complex, and there was considerably more scatter in the orientation determinations. However, the orientations of the fracture facets in these steels were clearly not {100}, but fell into two distributions, one near {211} and the other near {110}. Electron diffraction studies from the fracture surfaces indicated the presence of α′ and martensites in the type 304 but not in the type 310 cases; the possibility that this was responsible for the differences in fracture planes is discussed. Studies were also made of a type 304 specimen which had failed by SCC at 289 °C. No martensitic phases were detected at the fracture surfaces in this case, and the fracture facets were large and flat, similar to those for type 310. Cleavage-like fracture surfaces were also produced in type 304 steels by hydrogen embrittlement, using both gaseous hydrogen and cathodic charging, but the facets were too small for precise orientation determination. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly Professor of Metallurgy, University of Illinois.     

Solute carbon and nitrogen in the ferrite after continuous annealing of low carbon low aluminium steels

Low N_sol levels not only require reduced N contents (≤ 20 ppm) in steel but also Al/N ratios higher than 5 and C contents higher than 0.025% because of the synergic effect between N and C precipitations; scavenging of Al on nitrogen is far from complete even in steels coiled at 750°C. Low C_sol levels are achieved either at very lowC contents (C ≤ 0.003%) or at C contents higher than 0.025%. Due to high C_sol, carbon aging is expected in steels coiled at 750°C. Softer, bake hardenable steels with good aging resistance will be obtained in the range 0.025–0.030 % C independently of the coiling temperature. C supersaturation of the ferrite is reduced at higher cooling rates after continuous annealing. Changes in C and N supersaturation according to the steel composition and the continuous annealing process are explained, taking into account the carbides morphology and the annealing conditions.     

Time-resolved in-situ analysis of phase evolution for the directional solidification of carbon steel weld metal

Kinetic information about the phase transformation, during solidification process and solid-state transformation is essential to the material processing, such as welding. In our research group, in-situ phase identification system consisting of undulator beam and imaging plate have recently been used. The welding torch is driven by stepping-motor in the system. Those make possible that phase transformation can be identified in real-time under the condition of directional solidification and the spatial resolution of 100 × 500 μm. The time-resolution is 0.3125 seconds. In the present work, combination of analyzing method: the in-situ phase identification system, morphological observation by high-temperature laser scanning confocal microscopy and observation of microstructure at room temperature by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process. Phase transformation process of hypereutectoid carbon steel, during welding was analyzed as an example of combination observation.     

Investigation of the process of joining of constructional carbon steels

Conclusions In joining of carbon steels in the zone of former contact there occurs a white band which is tentatively identified as ferrite, which has a significant influence on the mechanical properties of the joint. The strength characteristics of the zone of joining of low- and medium-carbon steels reach the level of properties corresponding to a monolithic material with a deformation temperature of not less than 950°C and a degree of deformation of not less than 30%. The relative reduction is within limits of 2–15%. Fractographic analysis showed that fracture occurs in the ferritic band.The reason for formation of the ferritic band must be assumed to be preferential precipitation of excess ferrite in this area in cooling as the result of occurrence of micropores and and nonmetallic inclusions, which are embryos of the new phase. It was established that the ferritic band is distinguished by higher microhardness than the ferrite of the matrix of the steel.Translated from Poroshkovaya Metallurgiya, No. 4(328), pp. 69–72, April, 1990.     

Ni、Co元素对P92焊缝金属δ-铁素体形成的影响

运用大型热力学计算软件Thermo-CalcTM计算分析了P92钢焊接材料中分别添加Ni、Co元素时对焊缝金属中δ-铁素体形成的影响.研究结果表明,Ni、Co元素均能有效抑制P92钢焊缝金属中δ-铁素体的形成,对计算结果进行回归分析,提出了计算公式w(Ni)eq =w(Ni) +0.62w(Co),以定量评估Ni、Co元素对于δ-铁素体形成的抑制效果.综合考虑Ni、Co元素对焊缝金属Ac1点及δ-铁素体形成的影响,并结合Ni、Co元素的价格因素,认为在P92钢焊接材料设计中可以用Co部分替代Ni.     

磷对低碳钢坑孔腐蚀扩展的影响

采用特殊设计的闭塞腐蚀电池实验,并利用能量色散谱仪分析腐蚀产物成分,研究了磷对低碳钢蚀孔扩展的影响机理.结果表明:在模拟蚀孔的闭塞区钢样阳极表面沉积物中有磷酸根离子存在,在蚀孔活化腐蚀过程中,钢中磷转化成磷酸根离子是在金属表面进行的,并形成磷酸盐沉积在金属表面,对蚀孔的扩展起抑制作用;磷含量不同的钢,表面沉积物中磷酸盐及磷酸根离子含量可相差很大,从而对蚀孔扩展的抑制作用也会有明显差异;同一钢样中,由于磷的偏析,在蚀孔扩展过程中高磷部位腐蚀速率较低,低磷部位腐蚀溶解速率较高,磷的偏析带导致了凸凹腐蚀沟槽的形成.     

The role of hydrogen in the ductile fracture of plain carbon steels

In this report we consider the problem of hydrogen induced ductility losses in a plain carbon spheroidized steel. Specifically, the effect of internal hydrogen on the formation of voids from second phase (cementite) particles and their subsequent growth and coalescence was studied by careful microscopic inspection of uniaxially strained bars, both initially cylindrical and circumferentially notched, with and without hydrogen. Void initiation occurred with lower strains and stresses with hydrogen, although an equally important contribution to the ductility loss was from hydrogen accelerated void growth and coalescence. This latter process takes place by the propagation of voids along the grain, and possibly subgrain, boundaries which interlink the cementite spheroids. The results indicate that hydrogen facilitates interface separation, possibly by accumulating at the boundaries during hydrogenation of the specimen and lowering the cohesive strength, thereby making void initiation and growth along them easier.     

Effects of Zr on acicular ferrite formation in HAZ of Ti-bearing low carbon steels with high heat input of weld thermal simulations

To research the effect of Zr addition on inhibiting austenite grain growth of Ti-bearing low carbon steels,two steels with different Zr contents were prepared using a laboratory vacuum induction furnace. The performance of HAZ under weld thermal simulations was investigated. The impact toughness,microstructure and the second-phase particle performance of HAZ under weld thermal simulations were investigated. The HAZ toughness was improved from 13 J to 87 J by addition of 0. 010 % Zr into the steel,with the fracture mechanism changing from cleavage fracture to toughness fracture,which was mainly attributed to the second-phase particles that were potent to nucleate acicular ferrite in HAZ during welding. It was concluded that the second-phase particles TiO x + MnS,ZrO 2 + MnS or TiO x + ZrO 2 + MnS were nucleated on ZrO 2 or TiO x ( x =1. 5,2) . This method can be applied to grain refinement by promoting the acicular ferrite formation and growth during large-scale welding,as in the cases of thick steel plates requiring higher heat inputs during welding.     

Microstructures controlling the ductile crack growth resistance of low carbon steels

Microstructures controlling the ductile crack growth resistance in the ductile-brittle fracture transition region have been investigated with three low carbon low alloy steels, which showed characteristic differences in the R curves. The crack growth resistance is related to both the primary dimple morphology and the total length of local shear zones appearing on the fracture surface; the latter contribution predominates over the former. The heterogeneity of the microstructures, which constrains slip propagation at the grain boundaries, supplies sites for easy void nucleation and induces local shear and the resulting surface roughness.     

Fundamental studies on the deoxidation of low carbon steels with ferrosilicon

Laboratory size (4.6 kg) low carbon-iron melts were deoxidized using plain ferrosilicon, ferrosilicon with calcium aluminate flux, and ferrosilicon with calcium silicate flux. The dissolved oxygen and total oxygen contents in these heats were measured as a function of time and temperature using oxygen probes and quenched pin samples taken from the melts. The dissolved oxygen values, as measured by the oxygen probes, indicated that the iron-silicon deoxidation reaches equilibrium within five minutes of reaction time. However, the total oxygen (dissolved oxygen+oxygen in oxide inclusions), as measured from the quenched pin samples, took almost twenty minutes to reach a steady state. The two aforementioned features were common to all the experimental heats. Below 1978 K the ferrosilicon heats with calcium silicate or calcium aluminate flux had lower steady state total oxygen values as compared to the plain ferrosilicon heats; the difference was more significant at lower quenching temperatures. Also, below 1978 K, the dissolved oxygen-temperature relationship of flux heats was identical to their steady state total oxygen-temperature relationship, which indicated that the addition of fluxes leads to elimination of oxide inclusions. The performance of calcium aluminate and the calcium silicate fluxes in removing the oxide inclusions was found to be identical in these laboratorysize heats. U. B. PAL, formerly Senior Metallurgist, Melting and Primary Operations, Allegheny Ludlum Corporation, Technical Center, Brackenridge, PA 15014     

Influence of titanium and carbon contents on the hydrogen trapping of microalloyed steels

The presence of traps for hydrogen atoms influences the diffusivity and solubility of hydrogen itself in steels. In the present work, permeation measurements were carried out on hot-rolled microalloyed steels with different C and Ti contents in order to evaluate the number of irreversible and reversible traps. The number per unit volume of irreversible traps was correlated to calculated volume fraction of Ti(C,N) precipitates. These results, combined with microstructural investigations by transmission electron microscopy (TEM), showed that the largest number of irreversible traps was associated with steels having the largest volume fraction of fine and coherent Ti(C,N) precipitates. The reversible traps were associated with free Ti atoms, dislocations, and ferritic grain boundaries. Theoretical calculations confirmed the hydrogen binding energy of Ti free atoms (−27.1 kJ/mol).     

Effect of deoxidation practice and heat treatment on the hydrogen attack of carbon steels

The hydrogen attack (HA) kinetics of an electroslag refined (ESR) and a rare earth metal (REM)-treated steel in the Q. and T. condition were investigated by a highly sensitive dilatometer. Measured activation energies for bubble growth of 108 to 203 kJ/mol and pressure exponents of 0.9 to 1.6 are rationalized in terms of surface or grain boundary self-diffusion of iron as the rate controlling mechanisms depending on the external hydrogen pressure and temperature. Comparisons of the HA susceptibility of these steels with published work show that although the HA resistance of the ESR steel is not influenced by the heat treatment, the REM steel shows a significant decrease in the rate of sample expansion. SEM observations indicate that the improvement in the HA resistance of the REM steel is related to the presence of a very low density of methane bubbles.