International Journal of Metallurgical & Materials Science and Engineering (IJMMSE) ISSN 2278-2516 Vol. 3, Issue 2, Jun 2013, 45-52 © TJPRC Pvt. Ltd.

FORGING AND SINTERED CHARACTERISTICS OF DUPLEX STAINLESS STEELS PREPARED FROM 316L AND 430L POWDERS

R. MARIAPPAN1, R. TAMILSELVAN2, P. MATHIYALAGAN3, S. KUMARAN4 & T. SRINIVASARAO5 1,3Department of Mechanical Engineering, Vel-Tech Dr. R.R and Dr. S.R. Technical University, Avadi, Tamil Nadu, India 2Department of Automobile Engineering, Vel-Tech Dr. R.R and Dr. S.R. Technical University, Avadi, Tamil Nadu, India 4Department of Metallurgical and Materials Engineering, National Institute Technology, Tiruchirappalli, India 5Director, National Institute of Technology, Warangal, Andrapradesh, India

ABSTRACT

Four different duplex stainless steel samples were prepared from 316L and 430L pre-alloyed powders with and without Cr, Ni, Mo and Cu elemental addition. The green compacts were prepared using the universal testing machine under 560MPa pressure and then sintered at 1350°C for 4 hrs in dry nitrogen atmosphere. After sintering the sintered preforms were subjected to density, metallographical and mechanical properties evaluation. Sintered preforms exhibited density of 81.5- 83% theoretical density. Microstructure of sintered DSS revealed the lamellar constituents with austenite. To increase the density of the duplex stainless steels, forging operation was carried out at 1050 ± 10ºC. After forging the samples were kept in a muffle furnace for 30 minutes then subjected to subsequent water quenching. Forged duplex stainless steels exhibited density in the range of 94-96% theoretical density. Microstructure of the forged DSS exhibited the austenite with fine grains of ferrite. These forged DSS exhibited enhanced tensile strength in the range of 715 to 816 MPa and ductility in the range of 6-8.4%. These forged duplex stainless steels were found to be incorporated with added ferrite content.

KEYWORDS: Forging, Duplex Stainless Steels, Nitrogen Atmosphere, Ferrite Content

INTRODUCTION

Processing of materials through conventional powder metallurgy starts with the consolidation of metal or alloy powders by applying uniaxial or biaxial pressure followed by sintering. The density levels obtained in sintered samples are always much less than the theoretical values. Presence of such micro pores always makes the material weak because they act as sites of origination of cracks during service. Elimination of porosity in the sintered components calls for subsequent deformation processing of the preforms such as forging, extrusion, etc. Forging is accepted as economic and effective method of improving the density as well as the mechanical properties through promotion of homogeneous structure. Powder Preform Forging (PPF) has further advantages viz. optimum utilization of material, single blow finishing operation and isotropic properties [1–5]. The process of PPF involves subjecting the sintered porous preforms to either hot forging process or cold forging operations such as upsetting or repressing in closed dies. Elemental powders of metals or pre alloyed powders are used as the raw materials for the PPF process.

The final component in the forged conditions can attain properties superior to wrought materials. The quality of the product obtained through PPF is very much decided by the various process parameters such as forging temperature, initial preform density, alloying elements and the flow stresses [3–9]. Some of the limitations of hot forging are oxidation and decarburization of the surface of billets, excessive die wear, poor surface finishing and the induction of thermal stresses as a result of high temperature forging has been gaining importance in recent times [3,7,9]. Several researchers 46 R. Mariappan, R. Tamilselvan, P. Mathiyalagan, S. Kumaran & T. Srinivasarao have investigated the cold deformed ferrous and non-ferrous powder preforms and have reported their meritorious features such as good surface finish, higher accuracy, superior strength as well hardness due to work hardening and geometric hardening [3,7–16]. Stainless steel parts by PPF represent an important and growing segment of industry, through its characteristics such as enhanced corrosion resistance, oxidation resistance and good mechanical properties. Powder preform forged stainless steels found wide range applications in petrochemical industries [7&8]. In the present investigation four different duplex stainless steel samples were compacted and sintered at 1350±10°C for 4 hrs in dry nitrogen atmosphere. The sintered DSS samples were subjected to forging at 1050±10°C. The sintered and forged stainless steel samples were subjected to microstructural and mechanical properties evaluations.

EXPERIMENTAL PROCEDURE

Water atomized 316L austenitic and 430L ferritic stainless steel powders supplied by M/s Hoganas India Ltd. were used in the present work along with elemental powders such as Cu, Cr, Mo and Ni in different composition to produce four different duplex stainless steel samples. The chemical compositions of 316L and 430L powders with elemental combination of Cu, Cr, Mo and Ni mixtures i.e. DSS A, DSS B, DSS C and DSS D are given in table 1.

Schaffler’s diagram (figure 1) was taken into consideration while preparing the powder mixtures. Thus, CrE (Cr equivalent) and NiE (Ni equivalent) (CrE = %Cr +%Mo+ 1.5%Si + 0.5%Nb; NiE = %Ni + 30%C + 0.5%Mn) are obtained by introducing the wt% quantity of the corresponding element into the formula, which locate all the products in a well-defined area, at least in a theoretical point of view.

Table1: Chemical Composition of Various Grades/ Mixtures

Powder Flow Rate/ Apparent Cr C Ni Si Mn Mo Fe Grade 50 g (sec) Density g/cm3 316L 16.6 0.023 12.43 0.90 0.10 2.1 Bal 25.4 2.77 430L 16.56 0.012 --- 1.20 0.10 ---- Bal 25.4 2.88 DSS A (50%316L +50%430L) wt% Mixtures DSS B (49%316L +49%430L +2%Cu) wt% DSS C (45%316L+45% 430L +4 %Cr+3%Mo+3%Ni) wt% DSS D (44%316L+44% 430L +4 %Cr+3%Mo+3%Ni +2% Cu) wt%

Table 2 presents the chemical compositions of four different duplex stainless steel samples along with chromium and nickel equivalent (CrE and NiE) values derived from green composition. Cylindrical green compacts of 30mm diameter and 12mm height were prepared at a pressure level of 560 MPa. The green compacts were sintered at 1350°C for 4 hrs in dry nitrogen atmosphere. After sintering, the stainless steel compacts were cooled at the rate of 45 to 50°C per minute. Densities of both green compacts and sintered compacts were measured by mass and physical dimensions to understand the effect of chemical composition on densification. Some of the sintered stainless steels were kept at 1050±10°C for 2hrs in an electrical muffle furnace then were forged approximately at a pressure level of 100MPa at 1050±10°C by friction screw press.

Table 2: Chemical Composition of Duplex Stainless Steels

Elements Concentration( %wt) Composition Cr C Ni Si Mn Mo Fe Cu Nieq Creq DSS A 16.58 0.018 6.22 1.05 0.10 1.10 Bal --- 19.26 6.81 DSS B 16.24 0.016 6.09 1.03 0.10 1.08 Bal 2.00 18.87 6.62 DSS C 18.93 0.016 8.59 0.95 0.09 3.99 Bal --- 24.35 9.12 DSS D 18.59 0.016 8.47 0.92 0.09 3.97 Bal 2.00 23.90 9.00 Forging and Sintered Characteristics of Duplex Stainless Steels Prepared from 316L and 430L Powders 47

Forged and sintered DSS samples were subjected to microstructural analysis (structural changes) with the help of image analyzer. Structural evolution and phase transformation were studied by X-Ray Diffractogram with Cu-Kα target.

Mechanical properties such as tensile strength and hardness of sintered and hot forged duplex stainless steel samples were evaluated by using Hounsfield Tensometer and Rockwell hardness tester respectively. The ferrite content was measured by the magnetic method using Fischer ferrite scope.

Figure 1: Schaffller’s Diagram

RESULTS AND DISCUSSIONS Properties of Sintered Duplex Stainless Steels

The microstructures of the nitrogen sintered DSS samples are shown in figure 2. The microstructures consist of lamellar constituents with austenite.

The lamellar constituents are predominant in the DSS A and DSS C, but the copper containing DSSs such as DSS B and DSS D have slightly lower volume of lamellar constituents and more volume of austenite.

Copper being the austenitic stabilizer, it enhances the austenitic phase to the greater extent Higher magnification of nitrogen sintered DSS are shown in figure 3. The austenite and lamellar constituents are clearly analysed by SEM-EDS.

The SEM–EDS, reports that the lamellar constitution consists of chromium rich precipitates with ferrite. Moreover, chromium nitride precipitates are also identified along the grain boundaries.

Sands, et al. (1966) pointed out that 316L sintered in dissociated NH3 required a cooling rate of 200°C/min for preventing nitrogen absorption and precipitation of Cr2N [9&10]. Frisk, et al. (1992) determined that sintering of 316L in dissociated NH3 at 1250°C required cooling rates greater than 450°C/min [11].

The XRD patterns of DSS sintered in nitrogen atmosphere (figure 4) indicate the presence of ferrite, austenite and traces of Cr2N peaks. Ferrous oxide peaks are also present in the XRD patterns. 48 R. Mariappan, R. Tamilselvan, P. Mathiyalagan, S. Kumaran & T. Srinivasarao

Figure2: Microstructures of DSS Sintered in Nitrogen Atmosphere

Figure 3: Microstructure of DSS Sintered in Nitrogen Atmosphere a) at Higher Magnification, b) SEM- Micrograph

Figure 4: XRD Patterns of DSS Sintered in Nitrogen Atmosphere Forging and Sintered Characteristics of Duplex Stainless Steels Prepared from 316L and 430L Powders 49

Samples DSS B and DSS D have higher intensities of austenite peaks than the DSS A and DSS C owing to copper addition. The mechanical properties and ferrite content of sintered DSS are shown in the table 3. DSS A and DSS C have higher tensile strength and hardness than the copper containing DSS (DSS Band DSS D). Copper being an austenitic stabilizer it promotes more austenitic phase than the ferritic phase in the copper containing DSSs. Predominant phases of lamellar constituents in the DSS A and DSS C enhance the strength and lower the elongation. Especially, for the DSS C the addition of nickel in the stainless steel reduces the dissolution of nitrides [12], hence more Cr2N in that, enhances the tensile strength and reduces the elongation. Similarly, sample DSS C has more ferrite content than the other samples, where as the stainless steels with the compositions such as DSS B and DSS D have lesser ferrite content than the DSS A and DSS C.

Table 3: Mechanical Properties of DSS Sintered in Nitrogen Atmosphere

Compaction Pressure 560 MPa

Yield Strength U T S % Hardness Density (%) Ferrite Composition MPa MPa Elongation HRA g/cc Content DSS A 341 683 3.92 62 6.45 (82.7%) 20-24 DSS B 320 636 5.42 58 6.49 (83.1%) 18-22 DSS C 358 712 2.66 64 6.44 (81.7%) 28-32 DSS D 321 628 4.87 57 6.49 (82.0%) 22-24

The addition of copper in the DSS B and D slightly enhances the austenitic content there by reducing the ferrite content than their counterparts. The stress-strain curves for the DSS sintered in nitrogen atmosphere are shown in the figure 5.

Figure 5: Stress – Strain Curves for the DSS Sintered in Nitrogen Atmosphere

Hot Forging Characteristics of Sintered DSS

Sintered DSS were forged at a pressure level of 100 ± 10MPa at 1050°C. Table 4 shows the densities of green compact, sintered DSS, forged DSS along with the theoretical values. The forged densities vary between 14 to 15 % from the corresponding sintered densities. The microstructure of hot forged DSS (figure 6) consists of ferrite with austenite. Lamellar constituents in the sintered DSS are dissolved in to the matrix at 1050°C and biphasic structure is arrived [13]. Figure 7 shows the XRD patterns of hot forged DSS sintered in nitrogen atmosphere. The XRD patterns indicate only austenitic and ferritic peaks. 50 R. Mariappan, R. Tamilselvan, P. Mathiyalagan, S. Kumaran & T. Srinivasarao

Table 4: Densities of Hot Forged DSS Sintered in Nitrogen Atmosphere

Green Density Sintered Density Forged Density Theoretical Composition (g/cc) (g/cc) (g/cc) Density (g/cc) DSS A 6.12 (78.50%) 6.45 (83.00%) 7.431 (95.39%) 7.79 DSS B 6.15(78.74%) 6.49 (83.10%) 7.450 (95.39%) 7.81 DSS C 6.04 (76.74%) 6.34 (84.18%) 7.531 (95.69 %) 7.87 DSS D 6.19(78.25%) 6.49 (82.04%) 7.520(95.06%) 7.91

Chromium nitride peaks are absent as these chromium nitrides were dissolved into the matrix at 1050°C. Moreover the variations in the intensities of the austenitic and ferritic peaks depend upon the chemical compositions and cooling rate after forging. Samples DSS A and DSS C exhibit slightly higher intensities of ferrite peaks than DSS B and

DSS D. The mechanical properties of hot forged sintered samples in N2 atmosphere are tabulated in the table 5. Samples DSS A and DSS B exhibit higher tensile strength 816 MPa and 810 MPa respectively. Similarly DSS C and DSS D possess higher yield strength (468 MPa and 433 MPa) than DSS A and DSS B. Horng-Yih Liou, et al. (2001) reported that the mechanical properties depend upon the ferrite content.

Figure 6: Microstructures of DSS A a) Sintered Condition, b) Hot Forged Condition at Higher Magnification

The yield strength (YS) of DSS increases and the elongation decreases with the increase of ferrite content. However, with the increase of austenite, the yield strength slightly decreases whereas the tensile strength and elongation increase. This is explained by the fact that when the alloying elements of Creq are ferrite stabilizer, the ferrite content increases which promotes the strength of stainless steel. On the other hand

Figure 7: XRD Patterns of Hot Forged DSS Sintered in Nitrogen Atmosphere Forging and Sintered Characteristics of Duplex Stainless Steels Prepared from 316L and 430L Powders 51

when the alloying elements of Nieq are austenite stabilizer, the austenite content increases which reduces the strength of stainless steels. Besides, the ferrite formers also cause a decrease in the stacking fault energy (SFE) of stainless steels and make it difficult for the dislocation to climb and cross slip, which consequently results in an increase in the strength of the materials.

Table 5: Mechanical Properties of Hot Forged DSS Sintered in Nitrogen Atmosphere

Yield Tensile (%) of Forged Density (%) of Hardness Composition Strength Strength Ferrite g/cc Elongation HRA MPa MPa Content DSS A 7.531 (95.3%) 406 816 8.42 67 28 -32 DSS B 7.550 (95.1%) 415 810 7.04 66 26 – 30 DSS C 7.681 (95.8%) 468 765 5.48 71 38 -42 DSS D 7.661 (95.8%) 433 715 6.01 68 38 -40

On the other hand, austenite formers give rise to an increase in the SFE and make the dislocations easy to climb and cross slip[14]. Hence DSS A and DSS B are found to exhibit higher tensile strength and better elongation. The stress strain curves for the hot forged DSS sintered in nitrogen atmosphere are shown in the figure 8, from which it is clear that the yield strength, tensile strength and elongation of the hot forged DSS depend upon the chemical composition. DSS C and DSS D have lower tensile strength and elongations than the other duplex stainless steels such as DSS A and DSS B owing to the presence of higher ferrite content in the DSS.

Figure 8: Stress- Strain Curves for the Hot Forged DSS Sintered in Nitrogen Atmosphere

CONCLUSIONS

According to achieved results, Duplex stainless steels sintered in nitrogen atmosphere consist of lamellar constituents (chromium rich precipitates and ferrite) with austenite.XRD patterns of sintered DSS show the austenite, ferrite and traces of nitride peaks. Moreover, the addition of copper in DSS enhances the austenitic peaks in the compositions DSS B and DSS D.

The lamellar constituents in the microstructure improves the tensile strength and reduces the elongation in the duplex compositions such as DSS A and DSS C. Forged DSS are found to possess bi phasic structures such as ferrite and austenite. The lamellar constituents are dissolved at 1050°C. More ferrite content in the DSS C and DSS D lowers the tensile strength and elongation. 52 R. Mariappan, R. Tamilselvan, P. Mathiyalagan, S. Kumaran & T. Srinivasarao

REFERENCES

1. W.Lee Peter, A. Kuhn Howard, (1987) “P/M forging”, Metals handbook, ASM,; pp. 410– 417.

2. G.T.Brown, (1971); “The powder forging process: a review of the basic concept and development Prospects”, Powder Metallurgy, 14(27), pp 124–143.

3. W.J.Huppmann, G.T. Brown, (1978); “The steel powder forging process – a general review”, Powder Metallurgy, 2, pp.105–114.

4. Olle Grinder, Cao Young Jia, Ylva Nisson, (1979); “Hot upsetting and hot repressing of sintered steel preforms”, Modern developments in Powder Metallurgy, 15, pp. 611–637.

5. Tiyya Srinivasa Rao, (1991), “Densification behavior of aluminium and aluminium–copper powder sintered preforms of disc and ring shapes during cold deformation”, Ph.D. thesis, Bharathidasan University, Tiruchirapalli, India, pp. 1–10.

6. K. Jiten Dasa, P.S. Chandra, B. Misra, Sarma, (2008), “Hardness and tensile properties of Fe–P based alloys made through powder forging technique”, Materials Science and Engineering A, 479, pp. 164–170.

7. M. Abdel-Rahman, M.N. El-Sheikh, (1995), “Workability in Forging of Powder Metallurgy Compacts”, Journal of Materials Processing Technology, 54, pp. 97-102.

8. K.T. Kim, Y.C. Jeon, (1998), “Densification behavior of 316L stainless steel powder under high temperature”, Materials Science and Engineering A, 245, pp. 64–71.

9. Sands, R.L., G. F. Bidmead and D. A. Oliver, (1996), “The corrosion resistance of sintered stainless steels”, Modern Developments in Powder Metallurgy, 2, pp. 73-85.

10. C. García, F. Martín, M. P. De Tiedra, L. García Cambronero, (2007), “Pitting corrosion behaviour of PM austenitic stainless steels sintered in nitrogen–hydrogen atmosphere", Corrosion Science, 49, pp. 1718-1736.

11. K. Frisk, A. Johanson, C. Lindberg, (1992), “Nitrogen pick up during sintering of stainless steel”, Advances in Powder Metallurgy & Particulate Materials, Metal Powder Industries Federation, 2, pp.167-179.

12. J.J. Rawers, R. Bennett, R. Doan, J. Siple, (1992), ”Nitrogen solubility and nitride formation in Fe–Cr–Mn–Ni alloys”, Acta Metallurgical and Materials, 40 (6), pp. 1195-1199.

13. C.J. M´unez, M. V. Utrilla, A. Urena, (2007), “Effect of temperature on sintered austeno- Ferritic stainless steel microstructure”, Journal of Alloys and Compounds, 463, pp.552-558.

14. Horng-Yih Liou., Yeong-Tsuen Pan, Rong-Iuan Hsieh, Wen-Ta Tsai , (2001), “Effects of Alloying Elements on the Mechanical Properties and Corrosion Behaviors of 2205 Duplex Stainless Steels”, Journal of Materials Engineering and Performance, 10, pp. 231-24.