Stress corrosion cracking of high‐strength steels

The threshold stress intensity of stress corrosion cracking (SCC) in the NaCl solution, K_ISCC, has been measured for five low alloy steels. The effects of yield strength, alloy elements, microstructure and grain size on K_ISCC were studied. The results showed that K_ISCC decreased exponentially with increasing yield strength, σ_ys, i.e., K_ISCC = 1.38 · 10⁶exp(‐8.26 · 10^‐3σ_ys) for 40CrMoV steel and K_ISCC = 1.42 · 10⁶exp(‐4.66 · 10^‐3σ_ys) for 30CrMnSiNi steel. For low‐alloy high‐strength steels with σ_ys = 1400 MPa, the effect of alloy elements, microstructure and grain diameter larger than 7 μm on K_ISCC was little. The threshold stress intensity of hydrogen‐induced cracking during dynamical charging for 40CrMoTi steel decreased linearly with the logarithm of the concentration of diffusible hydrogen, C₀, i.e., K_IH = 31.3‐9.1lnC₀. This equation was also applicable to SCC of a high‐strength steel in aqueous solution, and in this case, C₀ is constant. The critical hydrogen enrichment concentration, C_th, necessary for SCC of high‐strength steel in water decreased exponentially with the increase in yield strength. It was possible to deduce the relationship between K_ISCC and σ_ys, i.e., K_ISCC = Ak₁exp(‐k₂σ_ys), where A = 3RT√πρ /2(1 + ν) , k₁ and k₂ are constants, which depend upon the compositions and microstructure of the steel as well as the test conditions.     

Computer simulation of multicomponent diffusion in joints of dissimilar steels

Diffusion in a typical joint of a stainless steel and a low-alloy steel has been simulated by means of the DICTRA software in a temperature range between 600 °C and 1200 °C. Calculated carbon concentration profiles are shown. A comparison with experimental data is made for a simulation at 650 °C. The simulations also predict how the amount of the various phases varies with the distance from the joint. The amount of carbon transported across the joint is evaluated as a function of temperature.     

Advances in the hot rolling of steels

Abstract

The 12th Conference and Exhibition of the Sheffield Metallurgical and Engineering Association (SMEA), held on 19 and 20 June 2003 at Stephenson Hall, University of Sheffield, attracted 96 delegates. The quality of the 20 presentations by recognised experts and discussions was of a high standard and provided an excellent insight into the progress being made in many aspects of rolling of both long and wide products. Dr Peter Morris (Swinden Technology Centre, Corus RD&T), Professor John Beynon (University of Sheffield) and Dr Ken Ridal (SMEA) review the highlights.     

热轧计算机管理与控制系统(上)

介绍宝山钢铁股份有限公司不锈钢事业部1780热轧工艺特点、对管理与控制的要求及其实现的方法,主要包括:炼钢、连铸、热轧一体化计划的编制思想和方法,即在一体化计划执行过程中板坯库的收发存管理、加热炉的装钢和抽钢顺序控制与管理以及碳钢、不锈钢混合轧制生产的模型控制思想及实现;质量管理的PDCA循环;在热轧与冷轧紧凑布置而导致热轧与冷轧前库之间物流极其复杂的情况下,钢卷库管理的特点及实现等。     

热轧计算机管理与控制系统(下)

3.3 板坯库对一体化计划执行的支持 连铸产出的板坯是炼钢终计划执行完毕的结果,这些板坯入库后作为进一步编制热轧初计划和终计划的对象材料.所以板坯的入库、堆垛、倒垛以及上料等仍以一体化计划为核心进行管理,其中堆垛的好坏直接影响到上料的作业效率.     

Thermodynamic calculations of the effective solidification range and its relation to hot cracking of aluminum-based ternary alloys

The correlation between experimental hot cracking (HC) and the calculated effective solidification range (ESR) (obtained as the difference between the formation temperature of a definite amount (65–90 wt %) of the solid phase and the nonequilibrium solidus temperature) is investigated by the example of Al-Cu, Al-Mg, and Al-Si binary systems. It is revealed that the location of the HB peak almost coincides with the calculated ESR peak. A good convergence between the calculated and experimental ESRs is established. The correlation between the HB and calculated ESR is, in general, considerably worse in the studied Al-Cu-Mg, Al-Cu-Si, and Al-Si-Mg ternary systems. It is shown that a correlation between the HB and calculated ESR that is similar to binary systems can be obtained for the radial joins of ternary systems if all alloys compared by the HB are crystallized by identical reactions with the participation of identical phases.     

Determination of the temperature range for deforming powder tool steels

Optimum temperatures for deforming powder tool steels are determined. It is established that the ductility limit during deformation of these steels in the dynamic recrystallization temperature region is higher than that at 1100–1150°C. Jet moulding technology followed by deformation in the temperature range established is shown to be promising.     

Initiation of stress corrosion cracking for pipeline steels in a carbonate-bicarbonate solution

The linearly increasing stress test (LIST) was used to study the stress corrosion cracking (SCC) behavior of a range of pipeline steels in carbonate-bicarbonate solution under stress rate control at different applied potentials. Stress corrosion cracking, at potentials below -800 mV(SCE), was attributed to hydrogen embrittlement. Stress corrosion cracking, in the potential range from about-700 to -500 mV(SCE), was attributed to an anodic dissolution mechanism. In the anodic potential region, the SCC initiation stress was larger than the yield stress and was associated with significant plastic deformation at the cracking site. The relative SCC initiation resistance decreased with in-creasing yield strength. In the cathodic potential region, the SCC initiation stress was smaller than the yield stress of steel; it was approximately equal to the stress at 0.1 pct strain(@#@ Σ_0.1pct) for all the steels. The original surface was more susceptible to SCC initiation than the polished surface.     

The effect of weld Heat-Affected zone (HAZ) liquation kinetics on the hot cracking susceptibility of alloy 718

A delay in grain boundary liquation was observed in the subsolidus portion of the weld heat affected zone (HAZ) of alloy 718 for the solution-treated material. However, for the homogenized or the homogenized and aged alloy, an instantaneous liquation of the grain boundaries occurred in the subsolidus HAZ. The above difference in the grain boundary liquation kinetics may account for the greater hot-cracking susceptibility of the homogenized or the homogenized and aged alloy compared to that of the solution-treated alloy. Existing models of grain boundary liquation are used to explain the observed kinetic effects associated with liquation in the subsolidus HAZ of alloy 718.     

热轧带钢厂板坯库及轧线工艺方案计算机动态模拟系统

通过微机Ｗｉｎｄｏｗｓ动画及后台处理，模拟了传统工艺方案设计阶段无验证的动态因素。应用结果表明，本系统能优化设计方案。     

Heat-insulating slag-forming mixture for tundishes for the casting of a wide range of steels

New heat-insulating slag-forming mixture P-4 has been developed and introduced for casting different sizes of semifinished products composed of a wide range of steels. The mixture has low contents of fluorine and carbon and contains additions of organic agricultural wastes. __________ Translated from Metallurg, No. 2, pp. 48–50, February, 2007.     

Effect of alloying elements on the cracking resistance of structural steels intended for low-temperature application

The cracking resistance of structural chromium-nickel-molybdenum steels are analyzed under delayed fracture conditions. The fracture surfaces of steel specimens and the effect of molybdenum and impurities (phosphorus, tin) on their cracking resistance are investigated by Auger electron spectroscopy and emission spectral microanalysis. The positive influence of molybdenum on an increase in the cracking resistance is shown to increase as its content increases to 0.6%; molybdenum does not influence the cracking resistance at its content of 0.6–0.98%, and this effect decreases at a molybdenum content higher than 0.98%.     

Quasi‐in‐situ observations of the sintering behaviour of molybdenum‐alloyed sintered steels

The sintering behaviour of molybdenum pre‐alloyed sintered steels was investigated. A scanning electron microscope with a hot stage attachment was used, enabling quasi‐in‐situ observations of the sintering process. The material MSP 1.5Mo (Fe‐1.5% Mo) sinters in the austenitic phase, while MSP 3.5Mo (Fe‐3.5% Mo) sinters completely in the ferritic phase, due to the increased molybdenum mass content. The significantly higher self‐diffusion coefficient of iron in the ferritic phase leads to the accelerated sintering of MSP 3.5Mo, compared to MSP 1.5Mo. The effect on the sintering behaviour by adding the alloying elements chromium and phosphorus was also studied. While phosphorus accelerates sintering processes, chromium decreases the sintering rate. A pronounced shrinkage during non‐isothermal heating, due to cooperative particle movement, was detected for both sintered steels.     

Physical properties in the temperature range from –180 to 400°C of nickel alloy steels for low-temperature applications

Nickel, which is the second neighbour to iron in the periodic table of elements, has a significant effect on most of the physical properties of the steels studied here, with the nickel content of the steels varying between 3 and 9%. Density and coercive force increase as the nickel content in the steel increases. By contrast, thermal expansion, modulus of elasticity and shear modulus decrease with increasing nickel content. Because of large scatter in the data, it is not possible to notice any significant influence of nickel content on Poisson's ratio and bulk modulus. Specific heat is also found not to vary with the nickel content of the steel. Electrical resistivity and thermal resistivity values increase, as expected, with increasing nickel content. The results of thermal conductivity measurements on 12 Ni 19 and X 8 Ni 9 have led to the development of modified Smith-Palmer relationships, which allow the calculation of the thermal conductivity values for the present set of steels and other steels with similar compositions.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.