Austenite grain growth modelling in weld heat affected zone of Nb/Ti microalloyed linepipe steel

Abstract

A model to predict the austenite grain size in an Nb/Ti microalloyed steel weld heat affected zone (HAZ) was developed. The present work investigates grain growth behaviour under the influence of pinning carbonitrides. The steel has been subjected to austenitising heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. The effect of temperature and heating rate on the grain size is studied. A model is proposed for the dissolution of NbCN precipitates. This model has been coupled to an austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the heating path simulating selected thermal profiles of the HAZ.     

Effect of silicon content on microstructure and toughness of simulated heat affected zone in titanium killed steels

Abstract

hree steels having different silicon contents were prepared to study the microstructure and toughness of the thermally simulated heat affected zone (HAZ) in titanium killed steels. For a low silicon addition, the oxygen content in the molten steels decreased remarkably. This in turn caused a change in the inclusion phase from predominantly titanium oxide to titanium nitride (TiN), the change being accompanied by two major microstructural modifications. The austenite grain size became refined and the quantity of intragranularly nucleated acicular ferrite decreased. The microstructural change was found to cause coarsening of Charpy fracture surfaces and deterioration of HAZ toughness of the steels. The minor change of silicon content therefore has a profound influence on the properties of titanium killed steels.

MST/1503     

Effect of precipitation on hot ductility of niobium and aluminium microalloyed steels

Abstract

The precipitation reactions occurring in C–Mn–Al and C–Mn–Nb steels before and after hot deformation have been examined and their influence upon hot ductility is discussed. Precipitation has been studied at 850°C, when ductility is poor, and also at 1100°C, when the ductility is good. Rapid intergranular precipitation occurred at 1100°C, but the precipitation present before deformation did not prove to be detrimental to ductility and grain boundary mobility at this temperature. Although only a limited amount of precipitation occurred at 850°C before deformation, intergranular precipitation continued during deformation resulting in embrittlement of the steels. At this temperature, strain induced transgranular precipitation of Nb(CN) occurred in the C–Mn–Nb steel and this is thought to be a major cause of poor hot ductility in this steel. By holding the steels for 15 min at 800–850°C before reheating to 1100°C, the area fraction of intergranular precipitation at 1100°C was increased. This produced a decrease in ductility at this temperature in the C–Mn–Al steel but had a less marked effect in the C–Mn–Nb steel.

MST/107     

细晶钢焊接热影响区晶粒长大及组织转变

分析了细晶钢的晶粒长大现象及其影响因素，讨论了ＨＡＺ的组织转变及其影响因素，提出了防止上贝氏体Ｂｕ和Ｍ－Ａ组元形成的有效控制措施。     

A continuum damage model for ductile fracture of weld heat affected zone

In this paper, the ductile plastic damage behaviour of weld heat affected zone (HAZ) is studied by use of continuum damage mechanics (CDM). Based on a continuum damage variable, D, the effective stress concept and the thermodynamics, a general continuum damage model for Isotropie ductile fracture is derived from a new dissipation potential chosen by the author herein. A comparison between the damage model and experimental results is presented and a good agreement is found. The model is also used to analyse the ductile plastic damage evolution in thermally simulated welding coarse-grained HAZ of a low alloy steel. The effects of stress triaxiality on plastic damage evolution and on ductile fracture of the coarse-grained HAZ are discussed.     

Post weld heat treatment of a niobium stabilized austenitic weld metal

A study has been made of the effects of post weld heat treatments in the range 700°C to 900 °C on the microstructural, impact and creep crack growth properties of a 16Cr-8NI-6Mn-Mo, V, Nb, B austenitic weld metal. These treatments result in the progressive decomposition of delta-ferrite to M₂₃C₆ and a phase as well as enhanced precipitation of NbC; the time- temperature- precipitation characteristics have been determined. Impact energies are severely reduced, whereas the creep crack growth resistance of material heat treated at 800°C or 850°C is significantly better than for the as-deposited weld metal.     

Process zone in fracture toughness testing of HSLA steels

Materials Science - The method of statistic fractography used after the fracture toughness tests provided the following information:     

Influence of aluminium on carbide precipitation in low carbon microalloyed steels

Abstract

Precipitation in an 0·1C–0·5Si–1·5Mn–0·15Mo–0·5Ni–0·05V–Fe(wt-%) alloy containing from 0·04 to 0·2 wt-%Al was examined in the tempered condition. After hot rolling, the steels were solution treated at 1200°C for 2 h, then quenched in water. Tempering was mainly carried out at 600 or 650°C for 1 h. The precipitates were identified and measured using transmission electron microscopy and energy dispersive X-ray analysis. It was found that coarsening of Fe₃C carbides in the prior austenite grain boundaries was associated with low Al content, while the size of Fe₃C particles in the lath boundaries was independent of Al content. It is proposed that Al segregated to the prior austenite grain boundaries during solution treatment, associated with vacancies, and decreased the rate of vacancy migration, thereby retarding the coarsening of cementite.

MST/1707     

T91钢焊缝及热影响区显微组织图象分析

T91钢具有良好的高温抗氧化性和抗腐蚀性,该钢含8％－9．5％Cr,合金含量复杂,焊接难度大,运用金相显微镜和扫描电镜（SEM）分析了不同爆接工艺条件下T91耐热钢焊缝及热影响区或区域显微组织特征,利用XQF－2000型显微图象分析仪对显微组织中各相的相对含量和奥氏体晶粒度进行了测量,分析了焊接线能量对T91钢焊接接头区组织性能的影响,结果表明,采用多道焊焊接工艺,严格控制焊接线能量在16kJ/cm左右,可以防止T91钢焊缝区奥氏体晶粒粗大,避免在热影响区出现声状铁素体组织,从而保证焊接接头区具有良好的组织性能。     

Effect of nitrogen content on simulated heat affected zone toughness of titanium containing thermomechanically controlled rolled steel

Abstract

The variation of simulated heat affected zone (HAZ) toughness with nitrogen content in titanium containing thermomechanically controlled rolledsteelwas investigated andinterpreted in termsof its microstructure and the amount of free nitrogen present. Measurement of the amounts of titanium and aluminium in precipitates extracted from the HAZ showed that 13–45%TiN and 72–79%AlN were dissolved at 1400°C peak temperature. The measured amount of titanium was in good agreement with the amount calculated by means of thermodynamic analysis, indicating that such analysis canbe used toestimate the amounts ofTiN and free nitrogen inthe HAZ at a given peak temperature. Simulated HAZ toughness was influenced notonly by the microstructure, in turn influenced by the amount of undissolved TiN at the peak temperature, but also by the amount of free nitrogen present after the formation of BN during the cooling cycle. Multiple regression analysis of the simulated HAZ toughness showed that the detrimental effect of free nitrogen was much greater than the beneficial effect of TiN in this experimental condition.     

Influence of vanadium on the static recrystallization of austenite in microalloyed steels

Torsion tests were conducted to study the static recrystallization of three microalloyed steels manufactured by electroslag remelting (ESR) with different percentages of vanadium, 0.043%. 0.060% and 0.095%, respectively, and approximately equal percentages of the other alloyforming elements. It was seen that, in contrast to niobium, dissolved vanadium has no influence on the activation energy. The influence only becomes notable when the precipitates start to form and the activation energy increases rapidly, thus inhibiting recrystallization. The critical temperature at which inhibition commences was measured as a function of the vanadium content and the deformation performed, and in all cases it was lower than the dissolution temperature deduced from the solubility products for nitrides, mainly because the testing conditions lacked thermodynamic equilibrium. Finally, a comparison was made of the microstructures obtained in two commercial steels, namely a C-Mn type steel and a vanadium microalloyed steel. Both were subjected to the same cycle of successive deformations, whose temperatures were lower than the critical temperature. After the last deformation, a much harder austenite was obtained in the microalloyed steel than in the C-Mn steel.     

Analysis of welded tensile plates with a surface notch in the weld metal and heat affected zone

The behavior of the plates with a surface notch under tensile external loading is experimentally and numerically analyzed. The specimens were made of micro-alloyed steel NIOMOL 490 K, welded by MAG (CO₂) welding method. Surface notches were machined by the spark erosion method in the weld metal and heat affected zone. A finite element analysis of the specimens subjected to the tensile external loading is performed, with a simplified treatment of the heat affected zone mechanical properties. Obtained results are compared with the results of the experimental investigation, and a satisfactory agreement for the specimens containing the weld metal notch is observed. However, results for the specimen containing a notch in the heat affected zone require further investigation, including the analysis of the effects of micro structural heterogeneity.     

Influence of grain size and precipitation on hot ductility of microalloyed steels

Abstract

The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a fine grain-refining precipitate, AlN reduced the hot ductility in the C–Mn–Al steel and delayed the onset of dynamic recrystallization. Coarser precipitates produced by raising the reheating temperature allowing dynamic recrystallization to occur gave improved ductility. For the niobium- and vanadium-containing steels, precipitate distributions which were in a coarse randomly precipitated form gave the best hot ductility. These occurred with the niobium-containing steel when heated to 1100°C and more generally in the vanadium-containing steel throughout a wide temperature range. The worst precipitate distribution occurred in the niobium containing steel when the NbCN was taken into solution before testing and reprecipitated in a fine form at the γ grain boundaries and within the matrix during the test.

MST/490     

Influence of material properties on dynamic fracture toughness of steels

Recent studies of plastic enclave formation at running brittle cracks were extended to account for the influence of crack tip boundary conditions on the temperature at which the enclaves start to develop. The En 2A and three other steels were used in the analysis. It was found that this temperature depends very strongly both on the magnitude and on the distribution of the stresses in the discrete crack tip zone. This suggests that the onset of enclave formation and the rate of their growth are governed by the balance of two sets of material characteristics. The first set consists of at least two parameters describing the microscopic fracture resistance which promotes enclave formation. The second set includes the macroscopic yield and flow properties which may make enclave formation more difficult in higher strength steels.These findings are related to the dynamic or crack arrest fracture toughness which is found to be derived from two different sources. One is connected with the microscopic plastic deformation of the fracturing metal in the crack tip zone and is present at all temperatures. The other is the result of enclave formation, it is present only at higher temperatures and is responsible for the energy transition. In contrast to the case of crack initiation, the dynamic fracture toughness depends not only on the microscopic fracture strength or strain but on the complete stress-displacement relationship of the weakened material which is governed by the microscopic fracture mechanism at the tip of a running crack. It is noted that the present results can be expected to be valid for all steels which fracture in the cleavage or quasi-cleavage modes.     

Study of cast microalloyed steels

Abstract

Improved grades of cast steels were prepared by microalloying C–Mn–Cr steels (0.15–0.22C, wt-%) with V (0.1)+Ti (0.01) or V (0.11)+Nb (0.034)+Ti (0.01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784–1078 MPa (80–110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0.9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the laths into polygonal ferrite occurred. Fine precipitates were deposited on the dislocation substructure.