Microstructure Characterization of Single and Multipass 13Cr4Ni Steel Welded Joints
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- Mokhtabad Amrei, M., Verreman, Y., Bridier, F. et al. Metallogr. Microstruct. Anal. (2015) 4: 207. doi:10.1007/s13632-015-0202-8
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13Cr4Ni martensitic stainless steels are frequently used in hydroelectric industries. Considering the size and geometry of the turbine runners manufactured in hydroelectric industries, multipass welding procedures are common methods for fabrication and repair. In this research, the microstructures and crystallographic textures of single-pass and double-pass welds have been studied as a first approach to understand a multipass weld. The highest hardness has been measured in the high-temperature heat-affected zone (HAZ) inside the base metal. Similarly, it has been found that the heat of the second pass increases the hardness of the previous pass and produces a finer martensite microstructure. In areas of the HAZ, 3–6 mm from the fusion line, a tempering-like effect is reported; traces of austenite have also been found in these areas documenting the complexity of the microstructure found in the multipass welds.
KeywordsMartensitic stainless steels Flux-cored arc welding (FCAW) 13Cr4NiMo steels As-welded microstructure Hardness Martensite δ-Ferrite Heat-affected zone Reversed austenite Tempering
Low-carbon martensitic stainless steels such as 13Cr4Ni steels are widely used in hydroelectric, power generation, offshore, and petrochemical industries, where high strength, toughness, and wear resistance of components are mandatory. Due to the size of the parts to be manufactured, welding processes are used for fabrication and repair despite the possible mismatch in mechanical properties of the body and the welded area . To minimize the mismatch, 410NiMo flux-cored filler wire is used to produce weld metals with sufficient toughness as it has the same composition and its carbon content is lower than conventional martensitic stainless steels .
During solidification, 13Cr4Ni steel solidifies to δ-ferrite and starts to transform into austenite at around 1300 °C. In thermodynamically equilibrium conditions, this transformation ends around 1200 °C . However, due to the actual high cooling rates of the welding procedures, small amounts of δ-ferrite can remain in the final microstructure at room temperature because of micro-segregation of alloying elements, during solidification . At lower temperatures, the austenite transforms to martensite with small amounts of retained austenite between martensite laths [4, 5, 6, 7]. Thus, after cooling down to room temperature, the microstructure is martensitic with potentially small amounts of δ-ferrite and austenite.
Welded parts of 13Cr4Ni are usually subjected to post-weld heat treatments (PWHT) which consist of a single- or a double-stage tempering heat treatment. The goal of these heat treatments is to temper the fresh martensite formed during cooling. It has been shown that a proper PWHT can transform some martensite back to austenite in large amounts (up to 25%) which influences mechanical properties . However, this study focuses on non-heat-treated weld conditions to better understand their microstructure formation and characteristics. In particular, the work will focus on an adjacent weld pass which produces a local heat treatment in the already-solidified regions.
Pre-heat temp. (°C)
Interpass temp. (°C)
Torch Speed (mm/s)
Filler deposit rate (kg/h)
Heat Input (J/mm)
Nominal composition of base metal and welding electrode (wt%)
5 cm thk.
1.6 mm dia.
The actual chemical compositions of the weld metal and base metal were measured by the glow discharge atomic emission spectrometers (except for C, N, O, and S that were measured by combustion/fusion determination methods). Microstructure, chemical composition, hardness, and austenite percentage were determined in the as-welded conditions. Samples were polished and electro-polished with a solution of 65 ml HClO4, 550 ml ethanol, 70 ml butyl-cellusolve, and 70 ml H2O at 25 °C, 25 V for 20 s to reveal austenite particles. The volume fractions of the austenite in samples were measured by x-ray diffraction from a Rietveld analysis Diffractometer at Institut de Recherche d’Hydro-Québec (IREQ) . Hardness evaluations have been done in the as-welded samples using a micro-hardness testing machine with a load of 200 g and a loading time of 10.2 s. A scanning electron microscope (SEM) operated at 20 kV was used to observe the microstructures of the samples. Electron backscatter diffraction (EBSD) maps were used to study the grain orientations of weld metal with Tango orientation map display and manipulation software.
Chemical compositions of base metal and weld metal samples (wt%)
Microstructure of the Base Metal After Welding
The base metal displays similar features for single- and double-pass welded samples. In the next sections, only the single-pass sample will be presented and discussed, but similar observations have been made on the double-pass sample.
The HAZ region in the base metal is the darkest area around weld. It etches easily as carbides formation could happen due to weld thermal cycle . Apart from becoming darker than the base metal when etched, there is no other detectable microstructure difference which can differentiate HAZ from base metal. Then it is difficult to define precisely the border between the unaffected base metal and HAZ. The HAZ seems 3 mm thick in Fig. 3, but it will be shown later that this distance is in fact about 6 mm based on hardness measurements.
Microstructure of the Single-Pass Weld Metal
In the single-pass weld metal sample, the microstructure can mostly be considered as fully martensitic. Although there are some evidence of inhomogeneities at the microscopic scale, such as former Widmanstatten austenite structures and δ-ferrite dendrite segregations traces, no austenite has been found using XRD measurements or electropolishing technique.
The contrast resulting from the Kalling’s etch is related to the various crystallographic orientations of the martensitic laths. These variations lead to different chemical activities in response to the chemical reagent and produce contrast in the metallography imaging. The large columnar microstructure observed macroscopically is actually the effect of a severe variant selection as most of the martensitic laths present in a grain have the same gray level, i.e., the same orientation. According to the various EBSD and Kalling’s etch images obtained in the present work, only few martensite orientations are formed from the austenite grain. Moreover, as martensite laths do not pass from a grain to another, the columnar microstructure represents the austenite grain when very few variant selection is taking place . Actually, the columnar-like martensite microstructure shows that severe variant selection has also been taking place during δ-to-γ transformation.
Microstructure of the Double-Pass Weld Sample
The double-pass weld sample consists of two adjacent weld passes onto the plate (see Fig. 1). The microstructure of the second pass is very similar to that of the single-pass specimen and will not be discussed in detail. Similarly, the base metal will not be discussed.
In order to characterize the local properties of the weld and relate them to the microstructure variations identified in the previous sections, hardness maps have been measured on the single-pass and double-pass samples.
On the other hand, the HAZ introduced by the second pass in the first pass spreads over a 3 mm length with a mean hardness of 355 ± 8 HV which is 5 HV higher than the original hardness of the single-pass weld. The heat-affected distance is similar to the one induced by the single pass in the base metal, but the effect on the hardness level is different. The hardness values in the heat-affected region of single pass are rather homogenously distributed, and it is not possible to distinguish the presence of a HT-HAZ. The HT-HAZ can actually be only found based on a microstructure observation (This is related to fine martensite formation in HT-HAZ explained in Figs. 15 and 16). After this 355 HV region, there is an area of lower hardness (320 ± 8 HV) which corresponds to the expected tempered region suggested by the microstructure examination (This is related to the darker region with the evidence of reformed austenite illustrated in Fig. 14).
No effect on the hardness is seen beyond 6–7 mm from the fusion line (the HAZ of the first pass corresponds to zone (A) in Fig. 20). The first pass thermal cycle tempers the martensite present in the base metal at distances between 3 and 6 mm. Microstructural evaluations showed that austenite particles were reformed in these regions where the Ac1 temperature of the base metal was reached. By moving toward the fusion line, the temperature reached by the weld thermal cycle is higher. The temperature increases until it becomes enough to transform some martensite back into austenite for a sufficient time to chemically stabilize it by gammagene elements diffusion.
The HAZ hardness increases at a distance less than 3 mm from the fusion line. In this region, the amount of reformed austenite is so high that no gammagene element enrichment is possible and fresh martensite is found at room temperature. Fresh martensite is responsible for the high hardness in this region. On the metallography sections (Fig. 10), it can be seen that the initial lath structure of the base metal martensite is replaced by an austenite parent grain-like structure.
The hardness reaches a maximum of 380 ± 11 HV at about 500 µm from the fusion line. This peak of hardness corresponds to the location where the temperature was high enough to transform all martensite back into austenite which led the microstructure to be fresh martensite after cooling.
Lower hardness was measured between the hardness peak and the fusion line (See zone (B) in Fig. 20). This corresponds to the region where δ-ferrite traces were found (Fig. 7). The very high temperature in this zone transforms some austenite into δ-ferrite, producing a slightly softer matrix. Retained ferrite is expected to be softer than fresh martensite, resulting in lower hardness.
In the weld, the hardness decrease continues for the first 1 mm (See zone (C) in Fig. 20). In this zone too, traces of δ-ferrite were found (Figs. 12, 13). These traces are δ-ferrite phases of the base metal which consumed partially in the semi-solid region of the weld metal and they had not enough time to mix thoroughly with the weld metal. These unmixed phases generate complex microstructures with a local composition that is an intermediate composition between the base metal and the weld metal. The rest of the weld at the left presents a relatively homogeneous hardness of about 350 HV which is a typical hardness of fresh martensite microstructure.
As for the effect of the double-pass weld, one can see that the heat effect of the second pass does not extend beyond 6–7 mm from its fusion line (zone A′ in Fig. 20). This is coherent with what was observed for a single-pass HAZ. A drop in hardness occurred at 3–6 mm away from the second pass fusion line. This drop is the tempering effect of the second pass similar to what has been found in the same distance from the fusion line of the first pass. The hardness of the first pass was increased of about 10 HV after receiving heat from the second pass in areas 3 mm from the fusion line and closer. In these areas, some fresh martensitic microstructure forms over the fresh martensitic microstructure of the first pass with a similar circumstance/mechanism to what has been reported in the single pass in the first 3 mm. This double-quenched martensite forms finer martensitic packets within the grains and more carbides ; then it is expected to be harder than a one-time fresh martensite.
The linear hardness profiles in Fig. 20 show that the second pass hardness is higher than the hardness of the as-welded single pass. The lower Mn content and the higher N percentages in the second pass could explain this difference. It can be predicted that a multipass procedure may improve the hardness uniformity of the welded joint if extending similar effects of the second pass to several passes.
If the decrease of the maximum hardness peak value is related to tempering effects induced by the second pass, the hardness increase on the first 3 mm of the HAZ of the second pass can be a consequence of the double quenching of the microstructure in this region. Double quenching is known to increase martensite dislocation density, promote carbides coarsening, and increase crystallography variant selections, which eventually produces harder martensite [18, 19]. The region that was originally fresh martensite as a part of the first pass weld metal is re-austenitized by the second heating/quenching sequence induced by the second pass. The second pass rises up the temperature higher than Ac1 at distances closer than 3 mm from the second pass fusion line resulting in a significant increase in hardness.
The present work showed that the second pass seems to homogenize the hardness of the built-up structure with a smoother transition between the base metal and the weld passes. However, one can wonder how additional passes could affect the microstructure and hardness distribution which will be presented by authors in a different article, and further research will be made in this direction.
The heat cycle of the weld produces heat-affected regions 6 mm deep into the base metal. The closest region to the weld line exhibits the hardness peak at its 500 µm from the fusion line. Traces of dendritic δ-ferrite were found, confirming that high temperatures were achieved during the weld deposition close to fusion line. Farther in the base metal, the heat cycle produces a microstructure which is the mixture of fresh martensite and tempered martensite from 500 µm to 3 mm. The regions farther than 3 mm from the fusion line exhibit the low hardness of the base metal-tempered martensite.
In 13Cr4Ni weld metal, columnar grain microstructure filled with fine martensite laths was observed. It has been observed that most of this columnar macrostructure from the first pass does not change by the heat cycle of the second pass. This finding which has been confirmed by the images produced from etched samples with Kalling’s no. 2 reagent revealed the same features as the maps from the EBSD. It has been shown that the heat of the second pass increases the mean hardness of the first pass but tempers the regions farther than 3 mm from its weld line resulting in softening of the previous pass HAZ. In some regions of the HAZ, the thermal cycle exposed by the second pass transforms the fresh martensite microstructure into austenite. The results showed that a multipass weld procedure produces a relatively more uniform hardness profile in the welded joint compared to a single-pass procedure.
The authors would like to acknowledge Natural Sciences and Engineering Research Council of Canada (NSERC), Institut de Recherche d’Hydro-Québec (IREQ), Alstom Power, and École de Technologie Supérieure (ÉTS) for the technical and financial support. The authors are grateful to IREQ laboratory for the Rietveld analyses and metallography studies and to Dr. Pierre Hovington for the SEM studies.