Effect of material inhomogeneity on fracture modes in aluminium–steel resistance spot welds

Resistance spot welding (RSW) is attractive for joining dissimilar materials, especially, aluminium to steel in automotive body. The direct joining of aluminium to steel forms an intermetallic compound (IMC) layer at their interface that dominates mechanical behaviour of the joint. A new formula was developed that considers material inhomogeneities such as the different mechanical properties in the weld such as base metal, heat affected zone (HAZ) and the weld nugget to accurately calculate the minimum weld nugget diameter required to enable pull‐out fracture. The shear strengths of weld regions such as the HAZ and IMC were directly measured and used as inputs to this new formula. The new formula was validated using experimental measurements from six combinations of aluminium–steel welds in comparison with analogous aluminium–aluminium welds. The new derivation was able to accurately predict fracture modes for all material combinations.     

Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACT

The present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Corrosion behaviour of innovative stainless steels in mortar

The main problem of the building sector is the limited durability of reinforcing concrete structures under highly aggressive environments due to the corrosion of reinforcements. In order to demonstrate that stainless steels are the adequate solution to avoid corrosion of reinforced concrete structures by contained chlorides, the corrosion behaviour of AISI 304 and AISI 316L stainless steels in mortar with two chloride doses is compared with the corrosion behaviour of three new stainless steels of low Ni content and reduced cost, and with the traditional carbon steel. The lowest chloride contamination (2% Cl⁻) has been used in this research to provoke corrosion in an active state of reinforcing carbon steel, whereas the highest one (5% Cl⁻) exceeds all expected levels found in the natural environment, including sea water. The new stainless steels remain in the passive state in mortar specimens with both chloride contents.     

模具钢材模拟切割系统的开发与实现

通过对模具钢材销售企业的调查分析,针对企业实际生产管理过程存在的下料方案不合理、人工选料费时等问题,提出了模具钢材模拟切割系统的需求分析及可行性分析.运用.NET开发平台和SQLServer2000数据库等软件,选用C#开发语言,开发了基于三层C/S模式的模具钢材模拟切割系统.通过设计绘图算法,采用.NET组件、数据库...     

C和N含量对V-N-Ti微合金非调质钢组织的影响

研究了C含量0.26wt%～0.42wt%及N含量0.0041wt%～0.021wt%变化对中低碳V-N-Ti微合金非调质钢锻态组织的影响.结果表明,C、N含量对V-N-Ti微合金非调质钢锻态组织中先共析铁素体的数量及分布存在显著影响.钢中的N含量一定,C含量约为0.33wt%时,组织中形成的晶内铁素体数量最多、组织最细小均匀;钢中的C含量一定,锻态组织中先共析铁素体的数量随着N含量的增加而增加,但N含量对先共析铁素体数量的影响要远小于C含量的影响.采用基于经典形核和长大理论模型计算所得析出动力学结果很好地解释了上述现象.     

Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels

Oscillating reaction rates have been observed in the steam oxidation of 21/4Cr-1MoNb and 9Cr-1Mo ferritic steels at 500–550°C. Changes in reaction rate are associated with the formation of a laminated, inner-oxide layer, made up of bands of fine and coarse-grain spinel oxide. The lowest reaction rates occur during growth of the fine-grain oxide. Coarse-grain oxide generally contains the same levels of Cr, Mo, and Si as the steel (after allowing for loss of Fe to the outer layer), while the fine-grain material contains three times these levels. Ni builds up in the metal and is present in the oxide as metallic particles (mostly associated with fine-grain oxide). A mechanism is proposed in which the highest reaction rates are controlled by diffusion of Fe ions through the oxide layer (as in normal parabolic oxidation) and the lowest rates by diffusion of Fe through the Ni-rich layer in the metal.     

Effect of aqueous LiBr solutions on the corrosion resistance and galvanic corrosion of an austenitic stainless steel in its welded and non-welded condition

The corrosion resistance and galvanic behaviour of a highly alloyed austenitic stainless steel (UNS N08031) and a nickel-base alloy (UNS N06059) in its welded (GTAW) and non-welded condition were analysed in LiBr solutions by means of electrochemical measurements. Samples microstructure was studied by SEM and EDX analysis. The alloys considered showed passive behaviour and they were able to repassivate after the breakdown of the passive film. The pitting resistance of the materials increased as the LiBr concentration decreased. With regard to galvanic corrosion, the welded metal was always the anodic element of the UNS N08031-welded metal pair.     

Fatigue performance of a SAE 1045 steel coated with a Colmonoy 88 alloy deposited by HVOF thermal spraying

The present investigation has been conducted to study the fatigue behavior of a SAE 1045 steel both uncoated and coated with a Colmonoy 88 alloy (NiCrBSiW) of approximately 410 μm thick, deposited by HVOF thermal spraying. Previously to deposition the samples were grit-blasted with alumina particles of approximately 1 mm in equivalent diameter. Tensile and fatigue tests were carried out with the uncoated and coated specimens. Fatigue tests were conducted under rotating bending conditions (R = − 1) at a frequency of 50 Hz. The samples tested were in three different surface conditions, including polished, grit-blasted and coated. The fatigue limit was determined by means of the staircase method employing a stress step of 5 MPa. The results indicate that the presence of the coating gives rise to a reduction in the fatigue life of the coated samples tested in air in comparison with the uncoated specimens. On the contrary, when the coated samples were tested in a NaCl solution at alternating stresses less than 350 MPa, these showed an increase in fatigue life in comparison with the polished uncoated samples. The analysis of the fracture surfaces of the specimens tested in air revealed that alumina particles present on the surface of the grit-blasted samples acted as stress concentrators, inducing the nucleation of fatigue cracks at the substrate-coating interface, which explains the reduction in fatigue life. However, under corrosive conditions and low alternating stresses, the presence of the coating provides an effective protection against corrosion-fatigue failures, giving rise to an improvement of the corrosion-fatigue performance of the coated system. On the contrary, at elevated alternating stresses, the coating was observed to delaminate from the substrate, leading to an impairment of the corrosion-fatigue behavior of the coated samples.     

Reheat cracking studies on simulated heat-affected zones of CrMoV turbine rotor steels

The reheat cracking susceptibility of the heat-affected zone (HAZ) of a CrMoV turbine rotor steel was investigated. Two base materials, one with a coarse-grain (155 μxm) and the other with a fine-grain (55 μim@#@) microstructure, were submitted to Gleeble HAZ weld simulations. Three peak temperatures were utilized: 1350,1150, and 950 °. Some samples were single cycled, and others were exposed to a double cycle. The samples that were double cycled experienced a second peak temperature 150 to 250 ° lower than the first peak temperature. The samples were then stressed in bending for different amounts and stress relieved under load to determine their reheat cracking susceptibility. All samples were metallurgically evaluated before and after the reheat cracking test. It was found that the prior-austenite grain size of the original base metal did not influence the reheat cracking susceptibility, but increases in peak temperature did. It was observed that the grain size and grain matrix microhardness that developed after the Gleeble cycles affected reheat cracking. It was found that reheat cracking did not occur when the microhardness was below 350 DPH and the prior-austenite grain size was less than about 80 μm.     

Corrosion behavior of engineering alloys in synthetic wastewater

The corrosion behavior of 1018, 410, and 800 steels exposed to synthetic wastewater have been studied using linear polarization resistance, cyclic potentiodynamic curves (CPCs), electrochemical noise (EN), and electrochemical impedance spectroscopy (EIS) tests. The conditions were: a biochemical oxygen demand of 776 ppm; a chemical oxygen demand of 1293 ppm; a pH of 8; and a cell temperature of 25 °C. From the CPC and EN results, no localized corrosion was found for the stainless steels. However, small indications of a possible localized corrosion process were detected for the 1018 steel. The EIS results revealed that different corrosion mechanisms occurred in the carbon steel compared with the stainless steels. The results show that the corrosion mechanism strongly depends on the type of steel. Overall, the 1018 steel exhibited the highest corrosion rate, followed by the 410 alloy. The highest corrosion resistance was achieved by the 800 alloy. In addition, scanning electron microscopy analyses were carried out to explain the experimental findings.