Lars-Erik Svensson

Microstructure of heat resistant chromium steel weld metals

The microstructure of weld metals of 2.25Cr−1Mo, 5Cr−0.5Mo, 9Cr−1Mo and 12Cr−1Mo type steels was studied with electron microscopy and atom probe field ion microscopy. Many different types of carbides and nitrides precipitated during welding and post-weld heat treatment (MC, M2C, M3C, M7C3, M23C6, MN, M2N). The eutectoid decomposition of retained austenite gave large aggregates of carbides which were found to be detrimental to the impact toughness of the weld metal.

Investigation of fracture and determination of fracture toughness of modified 9Cr-1Mo steel weld metals using AE technique

Abstract Three-point bending test and acoustic emission technique are used to determine the fracture toughness and fracture process of three types of modified 9Cr–1Mo steel weld metals. Scanning electron microscopy is used to accomplish fractography analysis of fracture specimens. Microstructure of weld metals is investigated using optical microscopy and transmission electron microscopy. The fracture process and factors which affect fracture of the 9Cr–1Mo steel weld metals in post-weld heat treated condition are studied. Experimental results show that the modified 9Cr–1Mo steel weld metals fracture by a quasi-cleavage mechanism at ambient temperature. The microstructure of the weld metals is composed of mainly tempered martensite with M23C6 precipitates. In weld metals, microcracks nucleate from non-metallic inclusions. Fractures develop very quickly when cracks started to propagate. Comparatively, in weld metals with low strength, microcracks initiate at a low stress, but propagation of cracks is limited by plastic deformation. In weld metals with high strength, microcracks nucleate at high stresses, but cracks propagate very quickly and lead to almost immediate fracture of the specimens. As a result, weld metals with the low strength have a higher fracture toughness, while weld metals with higher strengths has a lower fracture toughness.

Process Adjustments to Improve Fracture Behaviour in Resistance Spot Welds of EHSS and UHSS

In recent years new types of sheet steels, combining higher strength with better ductility have been developed. Some of these steels have shown limited weldability compared to mild steels. The hardness of the welds for these steels stretches from 380 HV for DP 600 to approximately 500 HV for TRIP (Transformation Induced Plasticity) and boron steels. Static peel tests of eight types of steels resulted in plug failures for the DP 600 steels, but some interfacial failures for higher strength steels. Generally, welds with a hardness exceeding 400–450 HV caused unstable fractures. To modify the hardness of the weld an in-process tempering of weld martensite was performed on a TRIP steel. Guidance to a proper tempering pulse was obtained through simulation of phase transformation and cooling of the weld. Welding experiments showed that the weld hardness could be reduced to approximately 350 HV, i.e. below the limit where interfacial failures start to occur.

Microstructure and properties of high strength weld metals

The recent investigations about high strength manganese – nickel alloyed weld metals are reviewed. The mechanical properties from different alloying concepts and the associated microstructures are compared. Interesting similarities regarding the tensile and impact strength is noted, while large variations in microstructure is found.

Evaluation of a temperature measurement method developed for laser metal deposition

Measuring temperatures in the material during laser metal deposition (LMD) has an inherent challenge caused by the laser. When thermocouples are radiated by the high intensity laser light overheating occurs which causes the thermocouple to fail. Another identified difficulty is that when the laser passes a thermocouple, emitted light heats the thermocouple to a higher temperature than the material actually experience. In order to cope with these challenges, a method of measuring temperatures during LMD of materials using protective sheets has been developed and evaluated as presented in this paper. The method has substantially decreased the risk of destroying the thermocouple wires during laser deposition. Measurements using 10 mm2 and 100 mm2 protective sheets have been compared. These measurements show small variations in the cooling time (∼0.1 s from 850°C to 500°C) between the small and large protective sheets which indicate a negligible effect on the temperature measurement.

Atom-probe investigation of precipitation in 12% Cr steel weld metals

Abstract The microstructure of two types of 12% Cr steel weld metals, one with the composition of a common 12% Cr steel and the other with a higher nitrogen content, was studied using TEM (transmission electron microscopy) and APFIM (atom-probe field-ion microscopy) in post-weld heat-treated condition. The microstructure of the 12% Cr weld metals consisted of tempered martensite, retained δ-ferrite, an irregular low-dislocation α-ferrite and precipitates. Precipitates in the weld metals were dominantly M 23 C 6 on different boundaries. Plate-like and fine cubic MN and M 2 N were found inside the α-ferrite. APFIM analysis showed that M 23 C 6 was almost a pure carbide and MN was almost a pure nitride. Carbon and nitrogen in the weld metals mainly existed in the precipitates. High nitrogen content did not change the composition of the precipitates, but increased the quantity of nitrides. Therefore, in the high nitrogen weld metal, the content of strong nitride-forming elements in the matrix decreased. These results are important in order to understand the strengthening mechanism of the high Cr steel weld metals, as well as of other high Cr heat-resistant steels.

Prediction of Hardness of Spot Welds in Steels

For modern steels, the hardness of the weld zone is often much higher than the hardness of the base material. This is due to the rapid cooling of the material after welding, in combination with the high hardenability, causing formation of hard phases like martensite or bainite. The hardness of the weld zone can be predicted by using well-known equations. By comparing the predicted hardness with measured data, it can be deduced whether the microstructure is martensitic or bainitic. It is found that the microstructure of the spot weld is martensitic for almost all steels, except for very lean alloyed steels. The hardness is dependent upon the chemical composition of the material. The hardness in the nugget zone may thus vary, depending on the steels that are welded together. If two similar steels are welded together, then the chemical composition of the nugget is well known. However, if two or more different steels are welded together, then the chemical composition must be estimated in some way. In this paper, some examples where different steels of different thickness have been welded are discussed.

Welding enabling light weight design of heavy vehicle chassis

Abstract Development of lightweight cars for saving fuel and reducing emission has been a priority for more than a decade. A similar trend is now seen for heavy vehicles. Here, however, the chassis rather than the cab is in focus, since this is by far the heaviest part of the vehicle. Using welding fabrication has many advantages like larger freedom in choice of material and more compact design. However, there are also factors like fatigue strength, residual stresses and geometric distortion, which must be addressed. There are large potentials to save weight in heavy vehicles by utilising high strength steels or aluminium alloys. In general, existing joining methods can be used, but new filler materials or recently developed post-weld treatments may be necessary to fulfil the demands on the components. In this paper, two examples are given, showing possible weight reduction solutions. In both cases, welding plays a central role.

Correlation between laser welding sequence and distortions for thin sheet structures

Thin ultra-high strength steel shaped as 700 mm long U-beams have been laser welded in overlap configuration to study the influence of welding sequence on distortions. Three different welding directions, three different energy inputs as well as stitch welding have been evaluated, using resistance spot welding (RSW) as a reference. Transverse widening at the ends and narrowing at the centre of the beam were measured. A clear correlation was found between the weld metal volume and distortion. For continuous welds there was also a nearly linear relationship between the energy input and distortion. However, the amount of distortion was not affected by a change in welding direction. Stitching and RSW reduced distortion significantly compared to continuous laser welding.

Neutron Diffraction Evaluation of Near Surface Residual Stresses at Welds in 1300 MPa Yield Strength Steel

Evaluation of residual stress in the weld toe region is of critical importance. In this paper, the residual stress distribution both near the surface and in depth around the weld toe was investigated using neutron diffraction, complemented with X-ray diffraction. Measurements were done on a 1300 MPa yield strength steel welded using a Low Transformation Temperature (LTT) consumable. Near surface residual stresses, as close as 39 µm below the surface, were measured using neutron diffraction and evaluated by applying a near surface data correction technique. Very steep surface stress gradients within 0.5 mm of the surface were found both at the weld toe and 2 mm into the heat affected zone (HAZ). Neutron results showed that the LTT consumable was capable of inducing near surface compressive residual stresses in all directions at the weld toe. It is concluded that there are very steep stress gradients both transverse to the weld toe line and in the depth direction, at the weld toe in LTT welds. Residual stress in the base material a few millimeters from the weld toe can be very different from the stress at the weld toe. Care must, therefore, be exercised when relating the residual stress to fatigue strength in LTT welds.