Quantitative analysis of complex carbo-nitride precipitates in steels

Two new types of extraction replicas for precipitate analysis in steels have been developed. These replicas, based on alumina, allow accurate quantification of the carbon to nitrogen ratios in carbo-nitride precipitates using parallel electron energy loss spectroscopy in the TEM. Experiments on microalloyed steels show the importance of the nitrogen content in promoting precipitation even at low temperatures.     

A computational study of austenite formation kinetics in rapidly heated steels

Surface hardening of steels involves rapid austenitization and subsequent quenching of the surface. The resulting extent of hardening largely depends on the rate of austenitization of the surface under the applied high heating rates. In the present work the kinetics of austenite formation in Fe-C alloys during rapid, non-isothermal heating conditions, characterized by high heating rates and short austenitization periods, were studied by means of computational simulation. Austenitization of lamellar pearlite/proeutectoid ferrite microstructures was simulated by assuming two kinetically distinct stages: i) dissolution of lamellar pearlite followed by ii) dissolution of proeutectoid ferrite. The two stages were simulated by two corresponding 1-D diffusion models employed in series. Numerical solution of the resultant moving-boundary diffusion problems provide calculated results regarding the dependency of vol. fraction austenite on thermal cycle parameters and on initial microstructural features of the steel. Analysis of calculated results showed that the vol. fraction of pearlite transforming to austenite during pearlite dissolution depended on maximum temperature, dwell time and pearlite interlamellar spacing. A functional relationship between these variables, consisting of a thermodynamic and a kinetic term, was established. On the other hand, the total vol. fraction of austenite forming in the steel, after both stages of austenitization, was found to follow a typical sigmoidal kinetic behaviour.     

射线检测技术在钢材尺寸测量中的应用

射线检测技术是一种重要的无损检测技术,目前在钢材的尺寸测量中得到了广泛的应用.简述了射线检测技术在钢材厚度测量、镀层测量和形状测量中的应用,并分别介绍了采用射线原理制作的普通测厚仪、镀层测厚仪和凸度仪的基本原理、工业应用和技术指标.     

Development and comparison of residual stress measurement on welds by various methods

Abstract

Residual stress constitutes an integral part of the total stress acting on any component in service. It is imperative to determine residual stress to estimate the life of critical engineering components, especially those that are welded. The stresses caused by non-uniform temperature distribution due to welding and the effect of these multiaxial stresses upon service performance are discussed. A controlled thermal severity test (CTS) was performed on mild steel plates bolted together, with anchor welds deposited on opposite sides. After cooling, bithermal and trithermal test welds were deposited one after the other. Varying welding stresses were deliberately introduced by using different thicknesses of both plates to change the thermal severity numbers (TSN). The main experimental technique used here to determine the magnitude and nature of residual stress is based on X-ray diffraction (XRD). It was utilised to develop and standardise other techniques. The XRD method is based on the peak shiftin the diffraction profile due to the presence of stress using a sin² ψ method. The peak shift is determined by orienting the sample at different angles ± ψ to the incident X-ray beam. The semidestructive technique of hole drilling and use of a strain gauge was also employed to determine residual stress in CTS specimens. The magnitude, nature, and direction of principal stresses were determined by relieving stresses through incremental blind hole drilling and measuring strain values at each step. The surface displacements arising due to hole drilling can also be determined by laser holography. A sandwich holography technique was developed to avoid unwanted rigid body motions of samples due to hole drilling when relieving stresses. Stress values were obtained by measuring fringe displacement between two exposures of a sandwich hologram, due to hole drilling. Results on the change in residual stress values with TSN are discussed. The residual stress values determined by XRD and sandwich holography were found to be comparable, and stress values obtained by hole drilling/strain gauge measurement were higher than these values. The reasons are discussed.     

Improving the Interfacial Microstructure Evolution of Ti/Stainless Steel GTA Welding Joint by Employing Cu Filler Metal

In the present work, Cu alloys as filler metals were applied to improve interfacial microstructural evolution during the process of gas tungsten arc (GTA) welding of TA15 titanium alloy with 18-8 stainless steel (SS). Results indicated that the interfacial brittle microstructure evolution of Ti/SS GTA welding joint was improved by adopting Cu alloy as the filler metal. Microstructure in the reaction zone (RZ) is composed of massive precipitations including brittle phosphide (Ti₃P), Ti₂Cu₃ and Ti(Cu, Fe) intermetallic compounds (IMCs), while the fusion zone (FZ) near SS is mainly composed of Cu-rich clumps and α-Fe+(Fe, Cr)₃P skeleton eutectic microstructure. Ti₂Cu₃ precipitations provided substrate for the heterogeneous nucleation of Ti₃P whose regular tetragonal and irregular morphologies are attributed to the proposed two kinds of forming mechanism. During the reaction of P with Ti₂Cu₃, the dropping of residual Ti₂Cu₃ contributed to the formation of macroporous in the middle of irregular Ti₃P precipitations. The dispersive distribution of massive brittle precipitations in the RZ leads to the fluctuant distribution of microhardness, which is up to about 700HV_0.5.     

Oxidation behaviour of some modern heat-resistant cast steels

The kinetics and morphological development of the oxidation of a selection of modern heat-resistant cast steels have been examined and compared with those of the traditional material, HK40. The materials examined had Cr contents of 24 to 29 weight percent (wt %), Ni contents of 30 to 46 wt %and in several cases minority additions of Nb, W, or both. One steel contained 3.3 wt % Al. Kinetics were measured gravimetrically over periods of 6 to 100 h and found to be parabolic in all cases except for the Al-containing steel, which oxidized in an irregular and irreproducible fashion. All steels formed an external scale of Cr₂O₃ with a Mn-rich spinel layer at the outer surface. Beneath this scale was a layer of alloy depleted in both Cr and Mn. Within the depleted layer inter-dendritic carbides had been destroyed, leaving either oxide near the external alloy surface or voids deeper within the alloy.     

Neutron diffraction measurements of residual stresses in a 50 mm thick weld

Residual stresses were determined through the thickness of a 50 mm thick ferrite steel weld plate using neutron diffraction. Whereas the limiting penetration depth for iron-based alloys is about 25 mm in the most typical neutron diffractometers, we significantly enhanced the penetration depth up to 50 mm with 2 mm spatial resolution by using the neutron wavelength of 2.39 Å. The selected wavelength minimizes the total neutron cross-section and beam attenuation, thereby, maximizes the neutron fluxes at depth. Two-dimensional mapping of the residual stresses shows that significant amounts of the tensile longitudinal stresses (over 90% of yield strength) were developed along the heat-affected zone of the weld.     

A design of experiments assessment of moisture content in uncured adhesive on static strength of adhesive-bonded galvanized SAE1006 steel

As part of a cooperative research program to develop and implement crash-resistant toughened adhesives targeted for future vehicles, this paper summarizes a study of the influence of pre-exposure of uncured adhesive and steel sheets in a humid and elevated temperature environment on quasi-static strength of bonded hot dipped galvanized SAE1006 steel joints.In this study, we use a DOE (design-of-experiment) program called DEXPERT to design the experiment and to analyze the effects of exposure temperature, exposure time, curing temperature and curing time on joint strength of adhesive-bonded galvanized SAE1006 steel. Prior to adhesive curing, the adhesive and galvanized steel coupons were pre-exposed to various relative humidity levels and temperatures. The experimental results were then analyzed by DEXPERT and the relative contributions of each factor on variance in joint strength were calculated. It was found that curing temperature is the most influential factor affecting the strength of adhesive-bonded galvanized SAE1006 steel joints. The curing of a joint at 180 °C can increase the robustness of the process and provides the greatest strength regardless of the variation of other factors. The joint strength curing at 150 °C shows a strong sensitivity to the curing time, while the adhesive cannot cure at 130 °C at all under all conditions. It has also been found that the pre-exposure of adhesive and steel for an hour can slightly decrease the joint strength at high temperature and humidity. Therefore, the effect of long time exposure of the uncured adhesive and steel still needs to be further investigated.     

New polymer/steel solution for automotive applications

Dry and wet steel/polymer adhesion was investigated using pull-off test and dielectric measurements. The influence of the composition of a hexafluorotitanic acid based surface treatment (Patent WO9607772A1) was also studied. It was noticed that when manganese or organic phase was missing from surface treatment, breaking force level decreases and steel surface was not protected from water effects. After ageing and drying, a complete recovery in breaking force was noticed except for treatment without the organic phase.