Scholarly article on topic 'Weld Metallurgy and Mechanical Properties of High Manganese Ultra-high Strength Steel Dissimilar Welds'

Weld Metallurgy and Mechanical Properties of High Manganese Ultra-high Strength Steel Dissimilar Welds Academic research paper on "Materials engineering"

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{"Ultra-high strength steels" / "laser beam welding" / "high manganese TWIP steels" / "martensitic stainless steels"}

Abstract of research paper on Materials engineering, author of scientific article — Martin Dahmen, Stefan Lindner, Damien Monfort, Dirk Petring

Abstract The increasing demand for ultra-high strength steels in vehicle manufacturing leads to the application of new alloys. This poses a challenge on joining especially by fusion welding. A stainless high manganese steel sheet with excellent strength and deformation properties stands in the centre of the development. Similar and dissimilar welds with a metastable austenitic steel and a hot formed martensitic stainless steel were performed. An investigation of the mixing effects on the local microstructure and the hardness delivers the metallurgical features of the welds. Despite of carbon contents above 0.4wt.% none of the welds have shown cracks. Mechanical properties drawn from tensile tests deliver high breaking forces enabling a high stiffness of the joints. The results show the potential for the application of laser beam welding for joining in assembly of structural parts.

Academic research paper on topic "Weld Metallurgy and Mechanical Properties of High Manganese Ultra-high Strength Steel Dissimilar Welds"

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Physics Procedia 83 (2016) 344 - 351

9th International Conference on Photonic Technologies - LANE 2016

Weld metallurgy and mechanical properties ofhigh manganese ultra-high strength steel dissimilar welds

Martin Dahmena'*, Stefan Lindnerb, Damien Monfort0, Dirk Petringa

aFraunhofer-Institut for Laser Technology, Steinbachstrasse 15, 52074 Aachen, Germany bOutokumpu Nirosta GmbH, 47807 Krefeld, Germany cPolytech Nantes, 44306 Nantes Cedex 3, France

Abstract

The increasing demand for ultra-high strength steels in vehicle manufacturing leads to the application of new alloys. This poses a challenge on joining especially by fusion welding. A stainless high manganese steel sheet with excellent strength and deformation properties stands in the centre of the development. Similar and dissimilar welds with a metastable austenitic steel and a hot formed martensitic stainless steel were performed. An investigation of the mixing effects on the local microstructure and the hardness delivers the metallurgical features of the welds. Despite of carbon contents above 0.4 wt.% none of the welds have shown cracks. Mechanical properties drawn from tensile tests deliver high breaking forces enabling a high stiffness of the joints. The results show the potential for the application of laser beam welding for joining in assembly of structural parts. ©2016 The Authors.Published byElsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Bayerisches Laserzentrum GmbH

Keywords: Ultra-high strength steels; laser beam welding; high manganese TWIP steels; martensitic stainless steels

1. Introduction

New ultra-high strength steels provide new lightweight opportunities and improve crash properties in future car bodies. By reducing the dead weight payload can be increased in vehicle construction for street and rails. Ultra-high strength steels with excellent deformation properties and intrinsic corrosion resistance are now commercially available. In order to utilize them different joining methods can be applied. As mechanical joining methods such as friction stir welding or self-piercing riveting are not applicable due to the high strength of the materials fusion

* Corresponding author. Tel.: +49-241-8906-307 ; fax: +49-241-8906-121 . E-mail address: martin.dahmen@ilt.fraunhofer.de

1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Bayerisches Laserzentrum GmbH

doi:10.1016/j.phpro.2016.08.036

welding should be applied with a strong limitation of the energy input. In the current contribution the effects of laser beam welding shall be investigated. Understanding the joining process as well as the response of the base materials on heat input is in order to create designs exploiting the full potential and fulfill the extensive requirements.

2. State of the art

The similar laser-welded TWIP (TWinning Induced Plasticity) steels joints exhibit a dendritic microstructure. Macrographs report a fully austenitic structure with grain coarsening in both, fused zone (FZ) and heat-affected zone (HAZ),where the latter is about 300p,m wide, see the studies of Dahmen, Daamen and Hirt (2014) and Behm et al. (2013). A few publications dealing with the laser welding ofTWIP steels to other grades are currently available. The main issue in these dissimilar welds is the appearance of chemical and/or phase inhomogeneities in the FZ since the multistage strain hardening ofTWIP steels strongly depends on the composition (Mujica et al. (2009)).

The welding suitability of a martensitic stainless steel (1.4034) in as-rolled as well as in press hardened condition and the mechanical properties of welded joints have been reported by Janzen et al. (2015). Fatigue test results were displayed for the case of hot stamped tailored blanks. The fatigue strength of the welded specimen, determined by Wohler tests, amounts to about 44% at 1T07 cycles compared with the fatigue strength of the base material. The results indicate a fatigue class of slightly above FAT 100. Quasi-static and dynamic tests according to the KS2 method reflect the behaviour of welds in hardened material for assembly. Load capacity and deformation are comparable to those of manganese boron steels. The scattering of the measurements ranges up to 9%. In all cases the joints failure mode is a brittle fracture in the weld zone. In as-rolled and in press hardened condition high hardeness at the fusion line, caused by bet martensite, requires a tempering treatment. For hot stamping this step can be. After hot stamping the heat-affected zone is transformed completely. Even the segregation lines are restored. A slight decrease ofhardness in the former high-temperature heat-affected zone and in the fusion zone indicates the presence of a weld. The weld zone shows an increased content of retained austenite and consequently a decrease in hardness.

As the 1.4034 the grade 1.4678 is a derivative of 1.4301 (304) where nickel is replaced by manganese. The steel is fully austenitic and exhibits strong work hardening by the TWIP effect (Graessel (2000)). The original yield strength of 500 MPa can be increased to up to 1100 MPa by cold forming. Material at low and middle strength level show an excellent welding suitability whereas welding becomes difficult at a strength above approximately 800 MPa (Lindner (2014)). The work hardening is lost in the fused zone but can be regained upon deformation (Lindner, Gerhards, Dahmen (2015)).

Experiments conducted by Behm et al. (2014) have demonstrated the formation of a martensitic phase in the dissimilar welds of TWIP HSD60 to ferritic S420MC. Depending on the mixing ratio of HSD600 into S420MC, more or less martensite appeared. This shall be explained by the shift of austenitic former concentration into the weld pool. In this study, the most efficient microstructure to be obtained with overlap welds offering maximum shear forces, was that with the largest austenite fraction in the joining plane. This is achieved through full penetration weld, welding from TWIP to S420MC sheet with a speed of 3 mmin"1. It was emphasized that mechanical shear strength of the dissimilar welds was not better than that of the weakest alloy.

Further studies on dissimilar butt joints TWIP Fe-22Mn-0.6C to a TRIP800 by Mujica et al. (2010) reported important segregation of manganese in the FZ and subsequent martensite formation. Manganese segregations in the form of C-Mn precipitates have also been reported along the dendrite boundaries in a TWIP/TRIP butt joint close to the TWIP side (Rossini et al. (2015)). Under tensile load, the latter butt joint fractured in the fusion zone. The resulting dissimilarjoints exhibited poor mechanical strength.

3. Experimental

The austenitic TWIP steel is a new 1.4678 with a manganese content of 16.5 weight percent cold worked to a yield strength of 1 GPa. Partner materials under investigation comprise a metastable austenitic steel 1.4301 (304) and a martensitic stainless steel 1.4034 (420) in press hardened condition. Table 1 shows the chemical composition of the three materials. All values refer to ladle analyses taken during production of the sheet metal. Sheet thickness is 1.1 mm in the case of the manganese steel, 1.5 and 2 mm for the chromium-nickel and the chromium steel, respectively.

During production the materials have undergone different treatments. The high manganese steel was cold rolled from its initial state at a yield strength of 500 MPa. The microstructure is fully austenitic. Cold rolling with subsequent annealing was applied at the austenitic stainless steel 1.4301. Base for the martensitic steel is a martensitic-ferritic stainless steel produced by cold rolling and annealing. Its microstructure consists of 94% austenite and 6% ferrite. For press hardening sheets were austenitised at 1150 °C, quenched in a mould, and tempered at 400 °C for 5 minutes. The resulting microstructure consists of martensite and approximately 28% austenite. Table 2 gives the standard values for the mechanical properties.

Table 1. Chemical compositions of the steels in weight percent.

Material C Mn Si S P Cr

1.4034 0.455 0.52 0.35 0.001 0.025 13.72

1.4301 0.04 1.4 0.5 0.012 0.04 19.1

1.4678 0.30 16.5 14.9

All welds were produced at butt joints. In order to limit the thermal load on the materials autogenous laser beam welding was applied for joining. All welding was conducted at room temperature with cooling at still air. In order to secure best achievable weld integrity as beam source a CO2 laser was used. The beam was focused by a mirror of 200 mm focal length onto a focal spot of 340 ^m diameter. Power was set to 2 kW at a feed rate of 4 m min"1, resulting in an input energy of 30 kJ m"1. As assist gas helium at a flow of 15 1 min"1 was applied. An argon flow of 20 1 min"1 was used for root shielding. Specimens were prepared by laser beamfusion cutting of strips of 30 * 80 mm2. No post-processing ofthe edges was applied.

Table 2. Mechanical properties.

Material Rm Rp0.2 A5 HB

1.4034 1800 -2000 950 - 1150 13 550

1.4301 500 - 700 >190 >45 <225

1.4678 1150 - 1300 950 - 1100 6-14

Inspection was started with a visual inspection of the welds with the aid of a stereo microscope. All welds without surface failures passed the examination. Metallography was carried out on macro and micro sections. Adler's reagent was used to reveal the structures in the 1.4678 similar welds and in the combination with 1.4034. An etching sequence featuring swabbing with Adler, water cleaning and drying, followed by a slight electrolytic etching (10% oxalic acid, 6V, 15s) was applied on the 1.4678/1.4301 combination.

a) Similarjoint 1.4678 b) Dissimilarjoint 1.4678/1.4301 c) Dissimilarjoint 14678/1.4034

Fig. 1. Macro-sections of the weldedjoints.

Hardness assessment was carried out using the method after Vickers. A proof load of 1 N was applied for 15 seconds. This gives well resolved indication of the hardness distribution without being disturbed by hard phases. Tensile tests were carried out at rectangular specimens on square butt joints in transverse direction. Force-elongation curves were registered. The tests were conducted load controlled at a deformation rate of 0.02 mm min"1.

In order to attain information on the effects of mixing to the local composition of the fused zone measurements by energy-dispersive X-ray spectroscopy were undertaken. As tracer elements iron, manganese, chromium, nickel, and silicon were defined. The carbon content was estimated indirectly. Based on a modified Schaeffler diagram the local microstructure was hypothetically predicted and tested by metallographic investigation.

4. Results

During visual inspection directly after welding no failures were observed. Also delayed cracking did not occur. In figure 1 typical macro-sections of all three welds are shown. All seams have a slight dumbbell shape as common for CO2 laser welds in thin sheet. With increasing difference in sheet thickness the asymmetry becomes more pronounced and the weld line becomes slightly curved.

Figure 2 shows two micro sections of the similar weld in 1.4678. The heat-affected zone in figure 2a is very small due to the low thermal conductivity of the TWIP steel (Lan (2016)). Over a width of 90 ^m slight grain refinement can be observed. Approaching the unaffected base materil an approximately 100 ^m wide zone with grain size equal to the base material, but with the cold working texture resolved follows (figure 2a from right to left). The fused zone is characterised by a dendritic microstructure (fig. 2b). By the etching pattern a slight segregation of manganese at the dendrite boundaries can be concluded.

a) Heat-affected zone and fusion line b) Weld centre line (scale 50 ^m)

(scale 100^m)

Fig. 2. Photographic images of the weld microstructures in the 1.4678 similar joint.

Mixing features become obvious in the weld of the combination 1.4678 and 1.4301. A photograph of the microstructure at the weld centre line is shown in figure 3a. Due to the increased carbon content in the mixed material the solidification appears to be columnar with dendritic substructure. In figure 3 b and c details of the microstructure at the fusion line of the 1.4301 are depicted. The microstructure of the fused zone appears fully austenitic. The solidification occurs as columnar with dendritic substructure. Compared to the similar joint, the microstructure has less preferential orientation and the dendrite spacing is smaller. At the fusion line of the 1.4301(fig. 3b) the equi-axially solidified: unmixed zone is visible. Adjacent in the fused material single alloy carbides segregated at the boundaries ofthe first layer ofcolumnar crystallites (fig. 3c).

Figure 4 shows a detail ofthe microstructure at the weld centre line in a dissimilar weld between 1.4678 and 1.4034. Besides columnar grains with dendritic substructure there develop equi-axed grains directly situated at the centre. In some grains a lath structure is visible indicating martensite. In figure 4b a micrograph of heat-affected zone is shown. On the not fused side ofthe fusion line a bright layer ofawidth ofl2 ^m is observed. As the thermal cycle is characterised by high temperature (above A3) and cooling rate this layer is considered to consist of untempered martensite. Figure 4c shows a detail with a visible partial molten, unmixed, and transformed zone. This martensitic area is followed by a region with partially hardened material over a width of 100 ^m at the center and 150 mm at root and upper bead. Between this zone and the base material an area with larger grains indicating an increased content of austenite can be seen in the cross section as darker area. Here a tempering of the hardened base material occurs.

As results of the hardness measurements the distribution of indentation hardness is displayed in figure 5. In case of a similar weld a decrease from average 480 HV0.i to approximately 300 HV0.i becomes obvious. This represents the hardness of the solution annealed material. The transition is smooth, no extremal values are measured. This indicates the loss of hardness, and hence, strength, by stress relieving through the welding heat. The total width of the heat-affected zone is indicated to about 0.3 mm. The hardness distribution of the weld 1.4678 to 1.4301 shows a straight transition from approximately 500 to 200 HV0.i between the materials. In the combination with the press hardened martensitic stainless steel a more complex pattern evolves. Hardness decreases from 480 HV01 to an average value between 302 and 400 HV01. This scattering is assumed to be an effect of mixing of the materials. On approaching the fusion line hardness values increase to about 700 HV01 followed by a decrease to less than 500 HV0.i in the soft zone. In the base material hardness is typical 600 HV01.

a) Weld centre line (scale 50 ^m) b) Fusion line at the weld top (scale 100 ^m) c) Fusion line (scale 20 ^m)

Fig. 3. Microstructure at the weld centre line and at the fusion line ofl.4301.

a) Weld centre line (scale 20 ^m) b) Heat-affected zone in 1.4034 (scale 100 ^m) c) Fusion line to 1.4034 (scale 20 ^m)

Fig. 4. Structure of the weld zone in the combination 1.4678 with 1.4034.

The hardness values and their distribution is reflected in the results of the tensile tests. They show large scattering with respect to the elongation at fracture (figure 6). In the similar combination 1.4678/1.4678 fracture and full elongation occur in the weld which is the weakest zone as shown by the hardness measurement. The graph in figure 6a show a characteristic curve for high-manganese steels. Yielding starts at approximately 17 kN and fracture occurs at forces above 40 kN. Recalculated yield strength amounts to 552 MPa. Hence, the weld shows the strength of the normalized material. In the case of the red curve the crack initiated at the weld centre line on the edge of the specimen. At the other specimens fracture was initiated at the fusion line and propagated diagonally through the weld to the opposite fusion line.

Fracture in the dissimilar weld of 1.4678 and 1.4301 originates at the fusion line of the austenitic stainless steel but deviates soon into the base material forming a ductile crack. All plastic deformation is found here which shows a

fracture strain of 25 to 36% and a necking of 10 to 26% in the base material. The red curve is not representative for the peak values because the specimen experienced some slip in the wedged clamps, but shows the same slope until failure. Yielding starts at 11.5 kN, fracture occurs at above 29 kN. Recalculated tensile strength is minimum 640 MPa, yield strength amounts to 256 MPa. The strength ofthis weld is defined by the weaker material.

naiisvLixi ^wiumai^ y/iiuii

Transversal coordinate y/mm

a) 1.4678/1.4678 b) 1.4678/1.4301 c) 1.4678/1.4034

Fig. 5. Hardness plots of thejoints taken 0.2 mm from the top and bottom surface, respectively.

0,40 0,60 0,80 1,00 Elongation Al/mm

a) 1.4678/1.4678

5,00 10,00 15,00

Elongation Al/mm

b) 1.4678/1.4301

0,10 0,15 0,20 Elongation Al/mm

c) 1.4678/1.4034

Fig. 6. Force-elongation curves of combinations welded obtained by transverse tensile tests. Numbers X.Y refer to combination (X) and individual members of the sample (Y).

Smallest elongation on fracture was detected for the combination 1.4678/1.4034. Fracture occurred at a force of 28 to 34 kN. The tensile curves indicate a brittle fracture. From a macroscopic assessment fracture starts at the weld centre line and propagates into the base material of 1.4034. Hardness measurements have indicated a lower strength of the fused material leading to yielding of the weld first. Deformation is blocked by the strong material of the 1.4678 and the martensitic layer on the side ofthe martensitic material. The reasons for crack deviation into that material is not yet understood. Micro cracks can be a reason but up to now there is no indication of their occurrence.

5. Discussion

Figure 7 shows a redrawn Schaeffler diagram considering the phase transformations observed in high manganese steels as developed by Klueh, Masiasz, and Lee (1988). The original Schaeffler diagram (dashed lines) are superimposed on the modified (solid lines) diagram. The point for the different alloys are symbolised as coloured dots. In the original diagram the grade 1.4678 is found in the fully austenitic region whereas in the modified diagram it should contain some percent of 8-ferrite. For the other two grades the original diagram has to be considered. The indication of 1.4034 is located in the austenitic-martensitic area with some 40% martensite in the as-rolled and 72% in the press hardened condition. The metastable austenitic grade is found in the austenitic-ferritic region with less than 10% ferrite. According to the correction by Lee et al. (2015) the indication if the alloy becomes situated in the austenite region.

---Original diagram / - Revised diagram / 91.4678 parent material 01.4034 parent material

Y / N / 01.4301 parent material

N / S / N / 1.4678 similar weld

N /1.4678 s / A 1.4678/1.4034 weld

s ml X /\ / Al.4678/1.4301 weld

' y+ 8 /

o s -A' Y + M N s / 1.4301 /

V. y' > ""

M / / / ' / / / / ," ' S x" s x ^ 6

5 10 15 20 25 30 Chromium equivalent/wt.%

Fig. 7. Standard and modified Schaeffler curve with an indication of the alloys under investigation.

As the composition of the fused material is considered to be defined by mixing of both partner materials it is expected their indication lies on the connecting lines. Table 3 lists the results from local EDS measurements, the figures are symbolised by triangles in the Schaeffler diagram in figure 7. There is, depending on the local mixing situation, a wide variation of alloying elements. Recalculation from the EDS measurements using the nickel and chromium equivalents show approximately the same trend. In the similar joints the actual composition shifts the phase distribution towards the ferritic and martensitic region (grey triangles upside down). For the grade 1.4678 welds with the partner 1.4301(upright grey triangles) the structure predicted can contain up to 10% D-ferrite and also traces of martensite. The decrease in nickel equivalent can be caused by the loss of manganese which can amount to up to 2% (Dahmen, Daamen, Hirt (2014)) or by dilution. Nickel supresses the formation ofmartensite but may occasionally lead to the segretation of carbides. In the weld to 1.4034 (white triangles) two measuring areas show a similar composition as the base material but shifted to the martensite region. Mixing with the manganese steel leads in one case to a structure containing austenite and ferrite. During metallographic inspection austenite was the dominating phase, in conjunction with 1.4034 also streaks with martensitic structure occurred.

Table 3. Results of local EDS analyses in the fused zone.

Partner Material C Mn Si S P Cr Ni Nieq Tr Phases

1.4034 0.4 3.53 12.87 12,50 12,87 y, M

0.35 10.41 17.90 11,00 17,90 y, M

0.38 6.43 13.69 11,90 13,69 Y

1.4301 0.15 9,25 0.40 0.012 0.04 17,15 4.59 9,59 17,75 Y, 5

0.3 7.55 0.58 17.48 4.41 13,91 18,35 Y

0.22 8.45 0.68 16.8 4.73 11,83 17,82 Y

1.4678 0.32 16.61 14.85 - 10,10 14,58 Y

0.32 16.05 14.5 10,10 14,50 Y

0.32 18.05 16.03 10,10 16,03 Y

The poor results of the tensile tests for the combination 1.4678/1.4034 are - hypothetically - caused by the discontinuous structure of the heat-affected zone. The presence of untempered body-centred tetragonal martensite introduces stresses and may lead to cold cracking. Heat treatment of the weld can especially improve the properties of the martensitic stainless steel. Pre-heating to martensite start temperature in conjunction with tempering at 400 °C was beneficial in order to improve the fatigue properties (Dahmen et al. (2015b)). The reaction of the austenitic high-manganese steel and especially its strength on such a heat treatment is currently not known. Mazancova, Ruziak, and Schindler (2012) have proven that high manganese steel Fe-0.18C-28Mn-2.3Al-0.98Si tolerates a heat

treatment at 500 °C for one hour without loss of hardness. At a dwell time of six minutes an increase in hardness was observed which disappeared on longer holding again. For the actual alloy of 1.4678 in cold worked state dedicated experiments still have to be conducted.

In order to understand the complex nature of fracture in the combination with the press hardened martensitic stainless steel a critical evaluation of the failure mode is required. This has to be done by meticulous metallographic inspection, fracture analysis and considerations of fracture mechanics. In this frame also the effect ofheat treatment can be studied and optimised. This will be the target of future work.

6. Conclusions

First tests on welding suitability of an austenitic stainless high manganese steel in similar and dissimilar joints have been carried out. The results show a general suitability delivering crack-free welds in thin gauge sheet material. For square butt welds the mechanical properties still have to be improved. Similar welds break in the fused zone through weakening of the material by the cast structure at the strength of the solution treated material. In the combination with the austenitic stainless steel the strength is determined by the weaker partner 1.4301. Fracture behaviour of the combination with the press hardened martensitic stainless steel 1.4034 is more complicated. Failure is initiated at the fusion line but the crack propagates into the strong base material of 1.4034. All fused zones solidify austenitic with martensitic streaks in regions with increased carbon content. Especially in the last case the results show the necessity of weld heat treatment in order to homogenise or to temper the heat-affected zone in the martensitic steel.

The results are not ready for application yet but show promising opportunities. Understanding the complex metallurgy as well as the resulting mechanical behaviour of the welds and utilising them for the production of reliably strongjoints by fusion welding will help to enable new constructions for eco-efficient applications.

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