Cracking behavior and mechanical properties of austenitic stainless steel parts

Short Communication

Cracking behavior and mechanical properties of austenitic stainless steel parts produced by laser metal deposition

J.Yu ?,M.Rombouts,G.Maes

VITO (Flemish Institute for Technological Research),Materials Technology,Boeretang 200,2400Mol,Belgium

a r t i c l e i n f o Article history:

Received 5July 2012

Accepted 31August 2012

Available online 8September 2012

a b s t r a c t

The cracking behavior,microstructure and mechanical properties of austenitic stainless steel parts pro-duced by laser metal deposition (LMD)are presented.The existing criteria for evaluating the solidi?ca-tion cracking sensitivity during welding of stainless steels have been adapted.Apart from the presence of sulfur and phosphorous,the presence of silicon was found to have a detrimental effect on cracking resistance.Cracking was not observed if the total content of sulfur,phosphorous and silicon was kept low enough,even not for stainless steels with austenitic solidi?cation mode.Three-dimensional parts produced using optimal process parameters and feedstock powder composition have been investigated in more detail.The parts have a density of 99.6%.The microstructure consists of ?ne columnar dendrites,which coarsen as a function of height along the building direction.This is accompanied by a decrease in hardness.The tensile strength and elongation are in general higher than for annealed wrought material.The tensile strengths are higher while the elongation is lower for samples loaded perpendicular to the build-up direction than for those loaded parallel.

ó2012Elsevier Ltd.All rights reserved.

1.Introduction

Additive manufacturing (AM)technologies produce three dimensional objects in an automatic process directly from a digital model by the successive addition of material,without the use of a specialized tooling.The process starts from digital data,most com-monly in the form of a three-dimensional computer aided design (CAD)?le,which is sliced electronically into a sequence of layers.For each layer two dimensional path information is de?ned and a program ?le for the AM machine is generated.On the AM machine a component is fabricated in a layer-by-layer fashion.In this study laser metal deposition (LMD)of stainless steel parts using a laser to melt powder transported by a coaxial nozzle is presented.This pro-cess is also referred to as laser engineered net shaping (LENS)[1,2],direct metal deposition (DMD)[3,4],laser solid forming (LSF)[5,6],laser cladding (LC)[7]and so on.Until now,laser metal deposition has been applied in many ?elds,such as aviation,navigation and automotive,for fabricating complex parts or repairing high-value components.

A wide variety of materials has been used for this process,including austenitic stainless steels such as AISI 316L and AISI 304stainless steel.During LMD the metal undergoes a rapid heat-ing and cooling cycle,which leads to high solidi?cation shrinkage stresses in the deposited layer.Austenitic stainless steels have a higher solidi?cation cracking susceptibility than low-carbon steel because of their higher thermal expansion coef?cient and lower thermal conduction coef?cient [5].

Most of the researches about solidi?cation cracking of austenit-ic stainless steels focused on conventional welding applications [8–12].Many of the studies focus on the selection of solidi?cation modes in relation to solidi?cation cracking resistance.Those modes can be divided into four types according to the solidi?cation behavior and the subsequent solid-state transformation [12].They are respectively austenitic A (L ?L +c ?c ),austenitic-ferritic AF (L ?L +c ?L +d +c ?c +d ),ferritic-austenitic FA (L ?L +d ?L +d +c ?c +d )and ferritic F (L ?L +d ?d ?d +c )modes.Different calculation rules have been de?ned to predict the solidi?cation modes based on the Cr-and Ni-equivalent of the material [13–16],as illustrated in Table 1.The ferrite stabiliz-ing elements such as Cr,Mo and Si are included in the Cr eq and sim-ilarly austenite stabilizing elements such as Ni,Mn and C are included in the Ni eq .It was found that the solidi?cation modes vary from A to F with increasing Cr eq /Ni eq ratio,as also presented in the Schaef?er [13]and WRC-92[15]diagrams.In relation to solidi?ca-tion cracking,it was recommended to select steels with F solidi?-cation mode to prevent cracking during welding.Pellini [17]found that the F solidi?cation mode provided a smaller critical temperature range for crack formation because of its smaller solid-i?cation temperature range than the A solidi?cation mode.Borland and Younger [18]concluded that the bene?cial effect of delta fer-rite could be attributed to its higher solubility for impurities than austenite,which consequently leads to less interdendritic segrega-tion and reduced cracking sensitivity.Hull [19]report that the low-

0261-3069/$-see front matter ó2012Elsevier Ltd.All rights reserved.https://www.doczj.com/doc/085027858.html,/10.1016/j.matdes.2012.08.078

Corresponding author.Tel.:+3214335696;fax:+3214321186.

E-mail address:jun.yu@vito.be (J.Yu).

er surface energy of the d–c boundary compared to d–d or c–c en-hances the interfacial stability and is an important factor for decreasing cracking susceptibility.

In addition,the presence of elements like sulfur and phospho-rous plays a signi?cant role in the cracking behavior of austenitic stainless steels.This is because these elements have a low solubil-ity in the major constituent of stainless steel and form low melting eutectic phases with iron,chromium and nickel at the interden-dritic region or along the grain boundaries[20,21].The segregation tendency at this location is high due to the wide solid–liquid range and low eutectic temperature[10].Therefore,in the evaluation cri-teria for cracking sensitivity of austenitic stainless steel the total content of sulfur and phosphorous should be included together with the predicted solidi?cation mode.This is accomplished by the Suutala diagram[22]and the improved Suutala diagram which is developed by Pacary for pulsed laser-beam welding[23].In addi-tion,Folkhard et al.referred that stainless steels with(P+S)con-tent60.02%,or(P+S)content60.03%and ferrite number P4, (P+S)content60.04%and ferrite number P8,(P+S)con-tent60.05%with ferrite number P12were not susceptible to solidi?cation cracking during welding[20].

The varestraint test is another practical and more direct way to evaluate cracking sensitivity,including transvarestraint test(TVT) and longitudinal varestraint test(LVT).The total crack length (TCL)got in LVT and brittleness temperature range(BTR)got by the maximum crack length(MCL)shown in TVT can be used for assessment.Lundin modi?ed the Suutala diagram on basis of the TCL from the varestraint test[24]and he speci?ed that for ‘TCL>2.5mm’,‘1.5mm

The LMD process differs from conventional welding in different aspects,such as the powder feeding and high solidi?cation and cooling speed.Song et al.studied the cracking mechanism during LMD of stainless steel[25].A higher sulfur,phosphorous and sili-con content was detected in the interdendritic regions,which re-sulted in solidi?cation cracking due to the separation of the liquid?lm under the action of high tensile stresses.

In the present paper,the cracking behavior of austenitic AISI 316L and AISI304stainless steel with different chemical composi-tions is studied.Appropriate solidi?cation cracking criteria,which should give guidance for materials selection for AM of austenitic stainless steel,are proposed.The microstructure and mechanical properties of parts fabricated by LMD are presented.2.Experimental setup

The LMD experiments were carried out using a7kW IPG?ber laser with out-coupling?ber with a diameter of600l m.The use of a focal lens with focal length of250mm and collimator lens with focal length of125mm results in a laser spot diameter of 1200l m on the substrate.The laser spot has a top-hat energy distribution.The powder is transported through a continuous coaxial nozzle(Fraunhofer-Institut für Lasertechnik)using argon transport and shielding gas.Samples were built on stainless steel AISI316L?at substrates with a thickness of8mm.In addition, SKM-DCAM software is used for generating the CNC programs with speci?ed tool paths.For each layer?rst scanning of the con-tour was programmed.Afterwards the interior was scanned using a raster deposition pattern,which was rotated90°between each layer,with0.6mm(50%)overlap between raster paths and 0.3mm overlap with the contour.For choosing appropriate pro-cessing parameters,preliminary experiments encompassing laser cladding of single lines,layers and3D parts have?rstly been car-ried out using different processing parameter combinations,fol-lowed by the analysis of interior quality of the specimens by checking their cross sections.The adapted experimental process-ing parameters used in this paper are listed in Table2,in which, P,V,D,F,g,Q s and hc are the laser power,scan speed,spot diam-eter,powder feed rate,overlap,shield gas?ow rate and layer thickness,respectively.

The chemical compositions of the powders referred in present study are listed in Table3.The last three are only used for comple-mentary analysis on the cracking behavior of austenitic stainless steels during LMD.

The microstructure was studied by optical and scanning elec-tron(Jeol JSM-6340F)microscopy.The samples were etched using a solution of hydrochloride acid,nitric acid and Vogel’s sparbeize. Electron Probe Microanalysis(EPMA,JEOL JXA-8530-F)using wavelength dispersive spectrometry(WDS)has been performed to analyze the local chemical composition.The mechanical proper-ties were determined by tensile testing(Instron5582)according to ASTM Standards(ASTM:E8/E8M-11).Three specimens produced under identical conditions have been subjected to tensile testing. The hardness was obtained by Vickers indentation measurements using a load of1kg.

Table1

Cr-and Ni-equivalent relationships for austenitic stainless steel.

Common name and authors Year Cr eq wt.%Ni eq wt.%

Schaef?er[13]1949Cr+Mo+1.5Si+0.5Nb Ni+0.5Mn+30C

DeLong[14]1956Cr+Mo+1.5Si+0.5Nb Ni+0.5Mn+30C+30N WRC-1992(Siewert)[15]1992Cr+Mo+0.7Nb Ni+35C+20N+0.25Cu HS(Hammar,Svennson)[16]1979Cr+1.37Mo+1.5Si+2Nb+3Ti Ni+0.31Mn+22C+14.2N+Cu

Table2

Experimental processing parameters(P,V,D,F,g,Q s and hc are the laser power,scan

speed,spot diameter,powder feed rate,overlap,shield gas?ow rate and layer

thickness,respectively).

Set P

(W)V(mm/

min)

D

(mm)

F(g/

min)

g Qs(L/

min)

hc

(mm)

1570750 1.2250%8.50.5

27501000 1.2 2.750%8.50.5

Table3

316L Stainless steel powders with chemical compositions.

C S P Si Mo Mn Ni Cr Fe

Nr.10.020.010.01 2.23 2.440.0811.4116.44bal.

Nr.20.0250.0110.030.5 2.5 1.412.717.1bal.

Nr.30.0190.0060.0170.53 2.1 1.410.416.8bal.

Nr.4

[26]

0.0720.0050.12 1.32 2.870.0614.4915.5bal.

Nr.5

[26]

0.0340.0020.0330.44 2.340.2414.3117.17bal.

Nr.6[6]0.0620.0120.0320.3400.888.2317.88bal.

J.Yu et al./Materials and Design45(2013)228–235229

3.Results and discussions

3.1.Cracking behavior

For powders1and2micropores and microcracks are present after LMD in contrast to for powder3(Figs.1–3).At a higher mag-ni?cation(areas1and2in Figs.2and3),it can be seen that the cracks are formed in the interdendritic regions.Micropores are nearly spherical in shape and are caused by the entrapment of gas in the melt pool[26].The porosity can be eliminated by increasing laser power.

It is known from literature studies in the?eld of welding that the solidi?cation cracking sensitivity of stainless steels depends on their chemical composition.The solidi?cation modes for the dif-ferent powders listed in Table3are calculated using different rules (see Table4).For a given powder the same solidi?cation mode is obtained using the different calculation rules.The powders1,2, 3and6are characterized by a ferrite–austenite(FA)solidi?cation mode,powder4by a austenite(A)solidi?cation mode and powder 5by a austenite–ferrite(AF)mode.Fig.4is obtained by considering the evaluation criteria of Suutala,Pacary,Folkhard and Lundin and the experimental observations.The powders that after LMD re-

2mm 2mm

(a) (b)

Table4

Solidi?cation modes for different powders predicted using different calculation rules.

Diagrams Materials Nr.1Nr.2Nr.3Nr.4Nr.5Nr.6 Schaef?er[13]Cr eq22.22520.3519.69520.3520.1718.39 Ni eq12.0514.1511.6716.6815.4510.53

Cr eq/Ni eq 1.84 1.44 1.69 1.22 1.31 1.75

Mode FA FA FA A AF FA WRC-92[15]Cr eq18.8819.618.918.3719.5117.88 Ni eq12.1113.57511.06517.0115.510.4

Cr eq/Ni eq 1.56 1.44 1.71 1.08 1.26 1.72

Mode FA FA FA A AF FA HS[16]FN539017 Cr eq23.127821.27520.47221.411921.035818.39

Ni eq11.87413.6711.23816.09215.139.86

Cr eq/Ni eq 1.948 1.56 1.82 1.33 1.39 1.87

Mode FA FA FA A AF FA 230J.Yu et al./Materials and Design45(2013)228–235

sulted in cracks (powders 1,2and 4)are marked in blue while the others (powders 3,5and 6)are marked in red.The following obser-vations can be made:

(1)The experimental results are not totally consistent with the

evaluation criteria applied for welding technology.The improved Suutala diagrams by Pacary et al.[23]and by Lun-din et al.[24]correspond better with the experimental observations than Suutala and Folkhard diagrams.Powders 2,3,4and 6are evaluated correctly.

(2)The experimental observation of cracking for powder 1is not

in agreement with the predictions presented in all the dia-grams.It should be noted that this powder contains more silicon than others.It is known that silicon can form low-melting eutectic phases such as Fe–Fe 2Si,NiSi–Ni 3Si 2and NiSi-c in the interdendritic region and along the grain boundaries [20,21],which greatly increases the cracking susceptibility.This is con?rmed by electron probe micro analysis (EPMA)of a LMD manufactured part using powder 2with normal Si content (0.5%).Even with normal Si con-tent,a signi?cantly higher Si content,apart from higher S and P contents,is detected inside the crack (point 2in Fig.5)than outside the crack (point 1in Fig.5),as shown in Table 5.Therefore,powder 1with much higher Si produc-ing cracks not as predicted illustrates the important role of

(d)

Improved Suutala diagram by Pacary, for laser welding

Suutala diagram, for conventional welding (a)

(b)

(c)

Table 5

Comparison of S,Si and P contents (in wt.%)inside and outside the crack for the part produced using powder 2(see Fig.5).Element S Si P Outside crack 0.0750.5920

Inside crack

0.552

1.185

0.008

J.Yu et al./Materials and Design 45(2013)228–235231

silicon:the presence of cracks at high Si content reveals that for this chemical composition the effect of silicon to result in low-melting eutectic phases is more decisive than the effect of silicon to act as a ferrite stabilizer.

(3)Powder 5did not result in cracks while according to its

chemical composition a high cracking probability is pre-dicted.The ratio of Cr eq to Ni eq for powder 5is only 1.3,which predicts a full austenitic solidi?cation mode.The high cracking resistance of this material is attributed to the low content of P,S and Si.Table 6shows the cracking behavior

Table 6

Cracking behavior of different powders related to impurity content.Powder number S +P +Si (wt.%)S +P (wt.%)N

content Cracking Fabrication method 1 2.250.020.047Yes Water atomized 4 1.4450.125–Yes Water atomized

30.5530.0230.062No Gas atomized 20.5410.0410.09Yes Gas atomized 50.4750.035–No Gas atomized 6

0.384

0.044

–

No

Gas atomized

(a)

(b)

(a) (b)

2

1

232J.Yu et al./Materials and Design 45(2013)228–235

of different powders related to impurity content of S+P+Si.

The cracking sensitivity decreases with decreasing S+P+Si content.

(4)A high nitrogen content is reported to be detrimental to

solidi?cation cracking resistance[9,11].Therefore,the N content of powders1,2and3was tested using instrumental gas analysis(IGA).The presence of cracks after LMD of powder2is attributed to the combination of relatively high N content(0.09wt.%)and of S+P+Si(0.541wt.%)content.

3.2.Microstructure and mechanical properties

In this section only analysis results of three-dimensional parts produced using powder3,which did not reveal any cracks after LMD,are presented.Three-dimensional samples of5mm height have been fabricated using the processing parameters listed in Table2.Fig.6a shows the cross section of the deposited part using parameter set1.The dendrites change growing direction consider-ably on the middle top of the melt pool due to a change in the heat ?ow direction.This area is remelted when using a higher heat in-put as shown in Fig.6b.

Fig.7a and b shows the samples,which were fabricated along respectively‘lying’and‘standing’orientations,that have been sub-jected to tensile testing.The loading direction is respectively per-pendicular and parallel to the build-up direction for Fig.7a and b.The densities of the samples measured by Archimedes method are in the range of99.5%and99.6%.

Fig.8illustrates the tensile test results with respect to yield strength,ultimate tensile strength and elongation.Values reported for cast and annealed wrought material are included as a reference [27,28].The mechanical properties are higher than for cast and an-nealed wrought material.The samples loaded parallel to the build-up direction(‘standing’orientation)have a lower yield strength, ultimate tensile strength and higher elongation.This trend is also reported in other studies[29].One of the contributing factors might be the orientation of the grain/dendrite boundaries.After LMD most of the dendrites are oriented along the building direc-tion.Upon loading parallel to the building direction into the plastic deformation region less barriers(dendrite/grain boundaries)need to be crossed by the dislocations[30].This is re?ected by a lower strength when loading parallel to the build-up direction.Another factor affecting the tensile strength is the size of grains/dendrites, as speci?ed in the Hall–Petch relationship:r s=r0+k y dà1/2,with r s the yield strength,r0a materials constant for the starting stress

for dislocation movement(or the resistance of the lattice to dislo-cation motion),k y the strengthening coef?cient(a constant unique to each material),and d the average grain/subgrain diameter [31,32].The dendrite spacing in the loaded region is larger for the sample loaded parallel to the build-up direction(Fig.7b).For this sample an increase in dendrite spacing from1–2l m up to about3–4l m is observed from bottom to top of the sample (Fig.9).This difference is also re?ected by the hardness measure-ments:the hardness near the bottom and top are respectively 205±10HV and174±10HV.The lower hardness is attributed to the lower cooling rate near to the top as a result of heat accumula-tion in the slender sample.For the sample loaded perpendicular to the build direction the cooling rate in the loaded region is larger and consequently a smaller dendrite spacing and higher strength are obtained.

Fig.10shows the SEM fractographies of the samples after ten-sile testing.Dimple morphology dominates on the fracture surface of all samples,which is indicative of a ductile fracture mode.In the dimples inclusions,which contain relative high amounts of oxygen, chromium,silicon and iron as revealed by Energy Dispersive X-ray analysis(Fig.10c),are present.The size of the inclusions is larger for the sample fabricated in‘standing’orientation:the inclusions are around0.8l m(Fig.10b)compared to0.3l m for the sample fabricated in‘lying’orientation(Fig.10a).The spacing between

[28][27][28] [27]

(a)(b)

J.Yu et al./Materials and Design45(2013)228–235233

the inclusions is larger for the sample fabricated in‘standing’ori-entation.Goodwin et al.[33]pointed out that the inclusion size has no effect in the ease of void nucleation in the weld material but the larger inter-inclusion spacing can result in higher ductility. This is consistent with the trend in mechanical strength.

4.Conclusion

The cracking behavior of austenitic stainless steel powders AISI 316L and AISI304processed by LMD was investigated.Cracks are observed in the interdendritic regions depending on the chemical composition of the powder.The experimental observations are not completely consistent with the cracking sensitivity criteria developed for welding.It was found that the silicon content was another critical parameter apart from sulfur and phosphorous to predict the cracking behavior.The total content of silicon,sulfur and phosphorous should be evaluated when assessing cracking sensitivity because of their tendency to form low melting phases in the interdendritic region.A high N content can increase cracking susceptibility in combination with a high S+P+Si content.

The microstructure after LMD of AISI316L consists of?ne columnar dendrites.A density of99.6%has been obtained.The hardness is about30HV higher and the microstructure?ner near the baseplate than at the top because of the higher cooling rate. The tensile properties after LMD are higher than for cast and an-nealed wrought material.Samples fabricated in‘lying’orientation show higher strengths and lower elongation than slender samples fabricated in‘standing’orientation.This is attributed to the differ-ences in dendrite/grain boundary orientation relative to the load-ing direction and in overall grain size as well as the inter-inclusion spacing.

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