Low-temperature Transformation Research Articles

In situ measurement of phase transformations and residual stress evolution during welding using spatially distributed fiber-optic strain sensors* *This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide

Welding of high-strength steels can result in large tensile strains as the base metal and filler material cool from their molten state. To combat these large tensile strains, low-transformation-temperature (LTT) metal fillers have been proposed. These fillers undergo a martensitic phase transformation at a lower temperature which can ultimately reduce the tensile strain or can even introduce compressive strain adjacent to the weld metal. However, the process for optimizing the composition of the LTT material, as well as various weld parameters for each unique weld geometry, can be quite expensive, especially if the acceptance criterion requires using neutrons, x-ray beams, or destructive techniques to characterize residual stresses. This work describes a simple, low-cost method for quantifying residual stresses and phase transformations in situ during welding. Spatially distributed fiber-optic sensors were bonded to a cast iron plate, along with tack-welded thermocouples, to measure temperature and strain during multiple passes with LTT filler metals. Results show that the fiber-optic sensors can successfully resolve compressive strain adjacent to the weld region caused by the martensitic phase transformations in the LTT filler material.

Reduction of distortion by using the low transformation temperature effect for high alloy steels in electron beam welding

Residual stress and distortion of welded specimens are issues when it comes to geometrical requirements. The surrounding material prevents the dilatation associated with transformation in the area of heat input resulting in residual stress and distortion due to thermal contraction. In the past few years, low transformation temperature (LTT) material was successfully used as filler wire to reduce residual stress as well as distortion in the weld seam in arc welding processes. High alloy Fe-based filler materials with levels of chromium and nickel ensure a martensitic transformation at reduced temperatures in a low alloy base material. The LTT properties counteract the accumulation of stresses due to thermal contraction with compressive stresses that develop within the transformed region. This work used a high alloy base material in combination with a low alloy filler wire resulting in a microstructure that shows the same properties as LTT weld metals. This in situ alloying allows for an alloy composition tailored to the process. In order to provide a point of reference, comparable welds were made using conventional high alloy filler wire. As a result, the distortion and longitudinal residual stress was significantly reduced compared to welding with conventional filler wire.

Comparative Study of VC, NbC, and TiC Interphase Precipitation in Microalloyed Low-carbon Steels

Interphase precipitation behavior of VC, NbC, and TiC during isothermal ferrite transformation in a series of microalloyed low-carbon steels with various amounts of carbide-forming alloying elements was comparatively investigated. Through the quantification by three-dimensional atom probe, the dispersion of alloy carbides formed by interphase precipitation was found to be higher in number density and smaller in size with more addition of carbide-forming elements or at lower transformation temperature. Moreover, although the dispersion of NbC and TiC was relatively finer than that of VC with the same amount of alloy addition, larger driving force for precipitation was necessary to obtain the same number density of NbC and TiC as that of VC. This deviation can be explained by the difference in amount of bulk alloy addition as well as interfacial energy between ferrite and alloy carbide, the latter of which was confirmed through the analysis based on Gibbs–Thomson effect.

Capability of martensitic low transformation temperature welding consumables for increasing the fatigue strength of high strength steel joints

Abstract The use of low transformation temperature (LTT) filler materials represents a smart approach for increasing the fatigue strength of welded high strength steel structures apart from the usual procedures of post weld treatment. The main mechanism is based on the effect of the low start temperature of martensite formation on the stress already present during welding. Thus, compressive residual stress formed due to constrained volume expansion in connection with phase transformation become highly effective. Furthermore, the weld metal has a high hardness that can delay the formation of fatigue cracks but also leads to low toughness. Fundamental investigations on the weldability of an LTT filler material are presented in this work, including the characterization of the weld microstructure, its hardness, phase transformation temperature and mechanical properties. Special attention was applied to avoid imperfections in order to ensure a high weld quality for subsequent fatigue testing. Fatigue tests were conducted on the welded joints of the base materials S355J2 and S960QL using conventional filler materials as a comparison to the LTT filler. Butt joints were used with a variation in the weld type (DY-weld and V-weld). In addition, a component-like specimen (longitudinal stiffener) was investigated where the LTT filler material was applied as an additional layer. The joints were characterized with respect to residual stress, its stability during cyclic loading and microstructure. The results show that the application of LTT consumables leads to a significant increase in fatigue strength when basic design guidelines are followed. This enables a benefit from the lightweight design potential of high-strength steel grades.

Low-Temperature Carbide Transformation in Pearlite of Medium-Carbon Steels

Metallographic studies of annealed medium-carbon steels (45, 40Kh, and 35KhGSA) showed that there is low-temperature peritectoid carbide transformation of pearlite of these steels, in which solid solutions of ferrite and cementite form solid solution with a broad homogeneity range on the basis of e-carbide Fe2C. It was suggested to plot the horizontal line of peritectoid transformation at the temperature of 382°C and the horizontal line of peritectoid transformation of cementite at 1650°C on the Fe–C diagram of state.

Solidification Cracking Assessment of LTT Filler Materials by Means of Varestraint Testing and µCT.

Investigations of the weldability of metals often deal with hot cracking, as one of the most dreaded imperfections during weld fabrication. The hot cracking investigations presented in this paper were carried out as part of a study on the development of low transformation temperature (LTT) weld filler materials. These alloys allow to mitigate tensile residual stresses that usually arise during welding using conventional weld filler materials. By this means, higher fatigue strength and higher lifetimes of the weld can be achieved. However, LTT weld filler materials are for example, high-alloyed Cr/Ni steels that are susceptible to the formation of hot cracks. To assess hot cracking, we applied the standardized modified varestraint transvarestraint hot cracking test (MVT), which is well appropriate to evaluate different base or filler materials with regard to their hot cracking susceptibility. In order to consider the complete material volume for the assessment of hot cracking, we additionally applied microfocus X-ray computer tomography (µCT). It is shown that by a suitable selection of welding and MVT parameter the analysis of the complete 3D hot crack network can provide additional information with regard to the hot cracking model following Prokhorov. It is now possible to determine easy accessible substitute values (e.g., maximum crack depth) for the extent of the Brittleness Temperature Range (BTR) and the minimum critical strain .

Characteristics of carbide-free medium-carbon bainitic steels in high-stress abrasive wear conditions

This study encompasses a comprehensive account of the abrasive wear properties of carbide-free, ultrahigh-strength bainitic steels processed through ausforming at three different temperatures well below the recrystallization stop temperature followed by bainitic transformation at temperatures close to the Ms temperature. Five medium-carbon, high-silicon compositions were designed for the study by suitably varying the alloying levels of carbon, vanadium, niobium, molybdenum, and aluminum. While ausforming at lower temperatures enabled a large number of nucleation sites leading to significant refinement of bainitic laths, the decomposition of austenite at relatively low transformation temperatures was accelerated due to the presence of a high dislocation density, thus enabling completion of bainitic transformation in a reasonable length of time. The steels were characterized in respect of microstructural features and mechanical properties, besides evaluation of wear resistance through a high-stress abrasive wear testing method with natural granite abrasives. The microstructures comprised different fractions of bainitic ferrite and/or granular bainite (56–68%), martensite (0–25%), besides a significant fraction of retained austenite (20–34%) manifesting as pools and also interlath films, depending on the ausforming conditions and subsequent cooling paths. A tensile strength of 1900 MPa level was achieved with hardness exceeding 500 HV for the medium-temperature ausformed steel containing a high carbon content that also showed lowest mass loss in the wear test. The hardness-to-mass loss ratio appeared highly promising with some of the carbide-free bainitic steels on par with or better than the reference martensitic steel. The high work-hardening capability as a consequence of the strain-induced austenite to martensite transformation was considered as the main factor for the superior abrasive wear resistance of the carbide-free bainitic steels.

High temperature superelasticity realized in equiatomic Ti-Ni conventional shape memory alloy by severe cold rolling

Great interests on high temperature shape memory alloys (HTSMAs) are recently raised, driven primarily by the aerospace and automotive industries, for their potential to serve as solid state actuators at high temperatures over 373 K. Near equiatomic Ti-Ni conventional SMAs, as the most applied SMA system, were considered to possess appreciable shape memory effect and superelasticity only below 373 K, which owes to low martensitic transformation temperatures. Here we utilized the martensite stabilization effect and successfully expanded the viable zone of B19′ (monoclinic) martensite to 616 K in a 35% cold-rolled Ti-50Ni (CR35) SMA. After a training cycle, it exhibited quasi-linear superelasticity of narrow hysteresis, ~3% superelastic strain and high strength over 1.2 GPa in a wide temperature range up to 483 K. It is the first exploration of high temperature superelasticity associated with the stabilized martensite. Using transmission electron microscopy and in-situ heating/tensile synchrotron X-ray diffraction, we reveal the origin of martensite stabilization and the underlying mechanisms of unique superelasticity in the CR35. Our work provides an attractively facile approach to realize high temperature superelasticity in Ti-Ni conventional SMAs, as well as in other kinds of SMAs, for precise actuation within an expanded range of working temperature and stress.

Fatigue strength of transverse attachment steel joints with single-sided arc weld using low transformation temperature welding consumable

This study investigated the applicability of low transformation temperature (LTT) arc welding consumables to improve fatigue strength against weld root failure. Transverse attachment joints formed by single-sided welding, similar to rib-to-deck connections in orthotropic steel bridge decks, were fabricated using an LTT welding consumable and conventional welding consumable. Fatigue tests were performed with out-of-plane bending loads using a vibration-type fatigue testing machine. The test results indicated that higher fatigue strength can be achieved in the joint with an LTT consumable compared with a conventional one. In addition, residual stresses around the weld bead were clarified by X-ray diffraction measurements and finite element analyses. The results revealed that the LTT consumable weld metal can reduce residual stress around the weld and also introduce compressive residual stress to the weld root, which can contribute to improved fatigue strength.

Low temperature transformation of lignocellulose derived bioinspired molecules to aviation fuel precursor over magnesium–lanthanum mixed oxide catalyst

Basic magnesium–lanthanum (Mg–La) mixed oxide materials with tunable interaction between Mg and La oxides were prepared for the low temperature production of green diesel and jet fuel intermediates via aldol condensation reaction. The prepared materials were thoroughly analysedby microscopic and spectroscopic techniques to elucidate its structural properties, including the determination of surface area and basic strengths. Among the tested materials, ML-3 catalyst reflects superior activity for CC bond formation between 5-hydroxymethylfurfural (HMF) and acetone, exhibiting >99% HMF conversion and 89% yield of the precursory C9 4-[5-(Hydroxymethyl)-2-furanyl]-3-butene-2-one product (HAc). Higher conversion and better product selectivity of ML-3 catalyst ascribe to the higher basic strength, moderate surface area and strong interaction between both the metals (Mg and La) in their mixed oxide forms. To understand the role of active sites, zeroth, first and second order kinetics were plotted, and a possible mechanism of CC bond formation was hypothesized. In order to rationalize the role of active ML-3 catalyst, HMF (and furfural) chain lengths were extended with different carbonyl compounds leading the formation of branched and cyclic chain (C9–C15) hydrocarbon precursors.

Comparison of effect of shot-peening with HFMI treatment or use of LTT consumables on fatigue strength of 1300\xa0MPa yield strength steel weldments

In this paper, T-joint samples in a 1300 MPa yield strength steel were produced using conventional or low transformation temperature (LTT) type consumables. The welded samples were either subjected to high-frequency mechanical impact (HFMI) treatment or to shot-peening. Fatigue testing was performed under fully reversed, constant amplitude bending load. Shot-peening gave a significant increase in fatigue strength for more than around 50,000 cycles. Shot-peened LTT welds had the highest fatigue strength, with conventional welds being shot-peened having slightly lower fatigue strength. HFMI treatment of conventional and LTT welds improved the fatigue strength also, but to a lesser extent, keeping the slope in the SN diagram close to three, while shot-peened samples had a slope of 5–7. Significant differences in compressive residual stress were seen between the different welds, with the most compressive stress found in the shot-peened samples. This was probably one of the main reasons for the improved fatigue life of shot-peened samples.

Challenges to rutile-based geoscientific tools: low-temperature polymorphic TiO2 transformations and corresponding reactive pathways

Rutile, a common accessory mineral in a wide variety of rocks, is the most stable naturally occurring TiO2 polymorph. The relationship between its trace element composition and formation conditions has provided geoscientists with discriminant tools for fingerprinting geological processes, such as magmatic evolution and subduction zone metamorphism, alongside applications to the study of sediment provenance. In the present work, volcaniclastic rock samples belonging to Fara and Saiq Formations, outcropping in Jebel Akhdar mountains, Oman, are studied with Raman spectroscopy and Electron Microprobe (EMP) aiming: of (i) the identification of different naturally-occurring TiO2 polymorphs, (ii) the evaluation of their trace element contents in relation with hydrothermal alteration features, and (iii) the analysis of the mineral reactive pathways behind the observed textural relationships. Raman investigations demonstrated that interstitial, fine-grained TiO2 corresponds to anatase, whereas rutile occurs as isolated single grains. EMP determinations further revealed that an identified Nb-enrichment in anatase is coupled with a corresponding Nb-depletion in rutile. The combination of the obtained results with petrographic observations enabled unravelling the TiO2 reactive pathways affecting the studied samples. Thus, a coupled polymorphic dissolution-precipitation reaction assisting rutile-to-anatase conversion has been defined, together with the role of Nb in further stabilizing the structure of the lower temperature polymorph. Semi-quantitative thermometric considerations suggest that rutile substrates are likely of magmatic origin, whereas anatase formation is clearly associated with a lower temperature aqueous environment. The gathered results raise fundamental questions concerning the application of commonly used rutile-based geochemical and thermometric tools.

Strengthening mechanisms influenced by silicon content in high temperature tempered martensite and bainite

Strengthening mechanisms influenced by silicon (Si) content during tempering of martensite and bainite between 500 and 650 °C were investigated. Microstructural features such as dislocation density, subgrain size, cementite size, and microalloy precipitate volume fraction and size were evaluated in low and high Si alloys heat treated to form bainite or martensite and tempered at 500 or 650 °C for 1 h. Higher Si contents increase the hardness of both bainite and martensite more after tempering at 500 °C, as compared to 650 °C. The increase in Si content leads to a greater increase in hardness in bainite compared to martensite after tempering at 500 °C. A strength model was used to calculate the increase in hardness from the quantitatively measured microstructural features affected by Si content. The strength model suggests that after tempering at 650 °C, increased Si content primarily increases the hardness by solid solution strengthening. However, after tempering at 500 °C, increased Si contents result in higher dislocation densities in martensite and bainite. A recovery model based on solute drag was evaluated and the results indicate that increases in Si content can delay recovery and result in higher dislocation densities after tempering at 500 °C. Increases in Si content also lead to finer cementite sizes in bainite after tempering at 500 °C. A cementite coarsening model was evaluated, and the results indicate that cementite refinement in bainite tempered at 500 °C cannot be accounted for by Si diffusion controlled coarsening alone. The lower transformation temperature employed to form high Si bainite, indirectly influenced by Si decreasing the martensite start temperature, is suggested to result in refinement of both cementite and subgrain size after tempering at 500 °C. Microalloy precipitation is not significantly affected by Si content in the present study.

A design method of tensile triangles and low transformation temperature weld metal for reduction of stress concentration and residual stress of welded joints

Stress concentration and residual stress have a significant influence on fatigue life of welded joints. In order to reduce the stress concentration of welded joints, a mathematical design method of tensile triangles (MTT) based on bionics was applied to weld shape design. Accordingly, the stress concentration of various weld beads in the corner boxing welded joint and the fillet welded T-joint was dissected using our in-house FEM software JWRIAN. It was found that there existed a large stress concentration in the conventional welded joints, whereas those welded joints with elongated weld bead were accompanied by a lower stress concentration, especially for elongated weld bead with MTT design. Furthermore, among the weld shapes of the corner boxing fillet welded joint, the rectangle shape of weld bead had the minimum stress concentration factor (1.05). For the fillet welded T-joint with MTT design, the stress concentration of weld toe decreased dramatically with the increase of the index of designed shape, but there was a minor difference of stress concentration at weld root between the weld beads with MTT design. In addition, application of low transformation temperature (LTT) weld metal utilizing martensitic transformation to the fillet welded T-joints can produce compressive residual stress at weld toe.

Control of Weld Residual Stress in a Thin Steel Plate through Low Transformation Temperature Welding Consumables

Low transformation temperature welding (LTTW) consumables are characterized by a low martensite start temperature and a large fraction of martensite forming in the weld. It can efficiently reduce the tensile residual stress because the volume expansion associated with the martensitic transformation compensates for the thermal contraction during cooling. In this work, a LTTW wire, designated as EH200B, was created for the arc welding of advanced high-strength steel thin plates. In comparison to conventional ER70S-3 wires, this LTTW wire generated an opposite distortion pattern. Neutron diffraction measurements along the center thickness of the welded plates showed the maximum residual stress along the longitudinal direction (LD) in the weld region, and the heat-affected zone (HAZ) immediately adjacent to the weld region was reduced from ~330 MPa to below 240 MPa by using the LTTW wire. A finite element (FE) model was developed to predict the residual stress distributions of the plates welded under these two wires. The simulation results showed reasonable agreement with the volume-average neutron diffraction data. Compressive residual stress in the weld region using the LTTW wire was predicted by the FE method. Electron backscattered diffraction and x-ray diffraction measurements confirmed ~90% martensite was present in the LTTW weld. The fatigue life of DP980 steel lap joint panels using EH200B wire nearly doubled that of ER70S-3 wire. This improvement was attributed to the high strength and low LD residual stress in the weld and HAZ immediately adjacent to the weld.

Surface- and volume-based investigation on influences of different Varestraint testing parameters and chemical compositions on solidification cracking in LTT filler metals

The subject of this study is how, and to what extent, Varestraint/Transvarestraint test results are influenced by both testing parameters and characteristics of evaluation methods. Several different high-alloyed martensitic LTT (low transformation temperature) filler materials, CrNi and CrMn type, were selected for examination due to their rather distinctive solidification cracking behaviour, which aroused interest after previous studies. First, the effects of different process parameter sets on the solidification cracking response were measured using standard approaches. Subsequently, microfocus X-ray computer tomography (μCT) scans were performed on the specimens. The results consistently show sub-surface cracking to significant yet varying extents. Different primary solidification types were found using wavelength dispersive X-ray (WDX) analysis conducted on filler metals with varying Cr/Ni equivalent ratios. This aspect is regarded as the main difference between the CrNi- and CrMn-type materials in matters of cracking characteristics. Results show that when it comes to testing of modern high-performance alloys, one set of standard Varestraint testing parameters might not be equally suitable for all materials. Also, to properly accommodate different solidification types, sub-surface cracking has to be taken into account.

Phenomenological kinetic model of the nano-bainitic steels to characterize the dynamics of the autocatalytic nucleation process

A kinetic model has been proposed in the present work, which formulates the dynamic nature of autocatalysis. The autocatalytic factor is quantified in the present work which involves interaction energy between bainite nucleus and stress field generated by existing bainite. This interaction energy provides the stored energy for autocatalytic nucleation and eventually reduces the nucleation barrier of bainite transformation. The change in Gibbs free energy due to carbon partitioning will also alter the autocatalytic nucleation with the bainite transformation. The model also evaluates the activation energy due to dislocation barrier, which is arising by solid solution strengthening of austenite. The slow kinetics at low transformation temperature is due to the larger activation energy of the dislocation barrier, which provides greater resistance to atomic displacement during transformation. The interaction energy between bainite nucleus and stress field generated by existing bainite was validated by determining the transformation shear strain exhibited in austenite which was found to be in the range as reported in the literature for displacive transformations.

AtomREM: Non-empirical seeker of the minimum energy escape paths on many-dimensional potential landscapes without coarse graining

Recently a non-empirical stochastic walker algorithm has been developed to search for the minimum-energy escape paths from the minima of the potential surface (Akashi and Nagornov, 2018; Nagornov and Akashi, 2019). This method is based on the Master equation for the distribution function of the atomic configuration which has a nature to seek the reaction path up along the valley of the potential surface. This paper introduces AtomREM (Atomistic Rare Event Manager), which is an MPI parallelized solver program package for executing this method, which yields reaction pathways in terms of the microscopic evolution of atomic positions. It is open-source and released under the GNU General Public License (GPL). A feature AtomREM interfaces with the LAMMPS Molecular Dynamics Simulator as a library of versatile potential functions for application to various systems. Examples of the applications to molecular and solid systems are presented. Program summaryProgram Title: AtomREM (Atomistic Rare-Event Manager)Program Files doi:http://dx.doi.org/10.17632/ys5mpgjfc8.1Licensing provisions: GNU General Public License 3Programming language: Fortran 90 and CSupplementary material: Result movies of simulation for argon solid and (CH2)4 moleculeNature of problem: AtomREM has been developed to help to find a reaction path on the atomic level using weighted Langevin mechanics on the inverse potential landscape. The method describes the low temperature transformation of complex systems without artificial forces or/and collective variables.Solution method: Recently [1] we designed a non-empirical scheme to search for the minimum-energy escape paths from the minima of the potential surface to unknown saddle points nearby with one dimensional application. The method is based on the Master equation and its solver algorithm is constructed to move the walkers up the surface through the potential valleys [2]. The stochastic algorithm uses a birth/death stochastic processes for numerous of walker in combination with the Langevin equation. Each walker obeys the statistics of average movement on the reaction path under biasing potential. Under this consideration the reaction path is derived as the average of the walker distribution from stable to saddle states.Additional comments: Program Github repository https://github.com/ryosuke-akashi/AtomREM

The Effect of Quenching Methods on Transformation Characteristics and Microstructure of an NiTiCu Shape Memory Alloy

Shape memory alloys are used in diverse fields. It is Ni–Ti alloys that are predominantly used for applications capitalizing on the functional properties of shape memory and superelastic effect. These properties are influenced by factors, such as alloy composition, alloying addition, thermal and mechanical treatment, etc. In any application, the transformation temperatures of the alloy/device are very critical. Moreover, the stability of transformation temperatures and a smaller hysteresis are preferred. Ni–Ti alloys have their own limitations, such as low transformation temperatures and lack of ductility. In order to overcome these limitations, researchers have used ternary additions, such as Zr, Hf, Cu, Pd and Au. The use of copper is practicable because of its lower cost as compared to Ti and Ni. Substitution of Cu for Ni up to ~ 25% in an equiatomic NiTi alloy causes it to exhibit SME. The transformation temperatures of NiTiCu exhibit less sensitivity to composition and a smaller thermal hysteresis. In this work, therefore, the effect of methods of quenching on transformation temperatures and microstructure of an Ni–Ti–4Cu (at.%) alloy has been explored. The material was cast using vacuum induction melting. It was solutionized at 950 °C for 2 h and cooled using different methods/quenchants: liquid nitrogen, water at room temperature, air, annealing (furnace cooling). The samples quenched by different techniques were characterized by XRD, DSC, OM, SEM and hardness test. The results indicated that the martensite plate size varied in samples quenched using different quenchants even though the morphology was the same in all specimens. In addition, the transformation temperatures and hardness values of the alloy specimens varied as the intensity of quenching varied. The results are presented and discussed in detail in the paper.

Investigating plasma keyhole welding with multiple wires for fusion welding with chemically graded weld seams

Fusion welding processes are characterised by high temperature gradients, caused by the local heat input, which in turn leads to the formation of thermal and transformation induced (tensile-) stress. If the resulting stress exceeds the yield point, distortion occurs, which has a strong influence on the geometrical precision of the component. To counter these effects, in the past few years Low Transformation Temperature (LTT) materials were successfully used as filler wire. Here, the martensite start temperature is reduced and the increased thermal expansion during the martensitic phase transformation is used to induce compressive stress. Thus the formation of tensile stress through thermal shrinkage can be actively counteracted by the formation of compressive stress at reduced temperature. This counteracts the thermal distortion with the use of phase transformation. However, a solid technical solution for locally inducing LTT injections needs to be found. The following article presents a new approach of generating chemically graded weld seams in order to change the microstructure from ferritic to martensitic LTT alloy within the welding process. Therefore, a multi-wire plasma keyhole process is used to create a new alloy in-situ with varying nickel and chromium contents. To obtain a chemically graded structure the wire feed speeds are changed continuously within the welding process. For the multi-wire plasma keyhole process, a conventional plasma welding torch is extended by additional wire feeders, developed by the RWTH Aachen University Welding and Joining Institute. The graded structure is achieved by combining a high chromium-nickel filler wire with a conventional low alloy filler wire. The length of the graded area is varied so that the transition from high to low alloy area is either long or short. The results show, that it is possible to establish a stable welding process and change the chemical composition within the weld seam in-situ. Cross sections show a changing microstructure from low alloy ferritic microstructure to a martensitic microstructure. The influence of the transition area on the resulting microstructure and mechanical properties, such as hardness, is investigated. The experiments were performed in order to develop a basic knowledge about the mixing of dissimilar materials in plasma keyhole welding and the effects on the microstructure.