International Iron & Steel Symposium, 02-04 April 2012, Karabük, Türkiye

680

COMPARISON OF MECHANICAL PROPERTIES OF AISI 1040, 420, 4140, 316 STEELS AFTER NITRIDING

Özgür ÇINAR

a, Burcu Nilgün ÇETİNER

a, Mehmet Masum TÜNÇAY

a, İsmail TOPÇU

a, Arif Nihat

GÜLLÜOĞLUa

aMarmara University, Dept. Of Metallurgy and Materials, İstanbul, Turkey, E-

mail:ozgur.cinar@marmara.edu.tr

Abstract Nitriding is a heat treating process used to create a hardened surface by diffusion of nitrogen into the surface of a metal. The three main methods used are: gas nitriding, salt bath nitriding, and plasma nitriding. The four different grades of steel – AISI 1040, 420, 4140, 316 Steels, respectively – were selected in order to compare the alteration of their mechanical properties after applying individually these three main nitriding methods. The specimens were carefully cut by an abrasive cutter, one of each specimen groups was reserved for metallographic examination before the procedures, then nitriding methods were applied to each group of specimens; afterward specimens were mounted, subsequently ground and polished. The microstructure was observed by optical microscope before and after each nitriding method. The hardness of specimens was measured before and after each treatment. Keywords: Gas nitriding, salt bath nitriding, plasma nitriding, AISI 1040, 420, 4140, 316 Steels, Mechanical

Testing Methods.

1. Introduction Traditional methods, such as nitriding treatments, have been applied to modify the surfaces of stainless steels and other ferrous materials [1]. The low processing temperature and low heat treatment distortion of the nitriding process have attracted considerable interest from a technological point of view, and a great deal of research has been performed to obtain high surface hardness by nitriding treatments [1]. Such thermochemical processes as plasma nitriding, gaseous and salt bath nitriding have been investigated and successfully used to produce a nitrogen expanded layer on the surfaces of various steels and thus to achieve combined improvements in surface hardness, wear resistance and corrosion resistance [2]. Physical vapor deposition can also be used for improving surface properties, but their application was often limited owing to poor adhesion of the coating [3]. Gas nitriding is an effective chemical heat treatment technique used to introduce nitrogen into metallic materials at certain temperatures and forms the hard nitrided layer [4]. Research results show that the surface microhardness of the nitrided steel can reach to HV1000~1200 and wear-corrosion resistance can be obviously increased [4]. Moreover, the shape of the materials can not be easily altered due to the low temperature of gas nitriding treatment [4]. Plasma nitriding is one of the most versatile nitriding processes with many advantages over the conventional salt-bath and gas nitriding [5]. The control of the metallurgical properties of the nitrided surface is the most important advantage of the plasma nitriding process, especially for high alloy steels [5]. In this process, parts are immersed in a nitrogen plasma environment, raised to a desirable temperature to facilitate diffusion of nitrogen into the bulk substrate until formation of the expected nitrides [3]. Advantages linked to plasma nitriding rest first on its versatility, coupled with its environmental-friendly aspects [3]. In Table 1, the compositions of steels used in the study are listed. The mechanical properties of these steels are compared in Table 2.

Table 1. Compositions of AISI 1040, 420, 4140 and 316 steels [6].

Steel C P Mn S Si Cr Mo Ni N

AISI 1040

0,37-0,44

0,040 (max.)

0,60-0,90

0,050 (max.)

- - - - -

AISI 420

0,15 (min.)

0,040 1,0 0,030 1,0 12,00-14,00

- - -

AISI 4140

0,38-0,43

0,035 (max.)

0,75-1,0

0,040 (max.)

0,20-0,35

0,80-1,10 0,15-0,25

- -

AISI 316

0,08 0,045 2,0 0,030 1,0 16,00-18,00

2,0-3,0 10,00-14,00

0,10

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Table 2. Comparison of mechanical properties of AISI 1040, 420, 4140 and 316 steels [7].

Steel Tensile Strength(MPa) Hardness (HB)

AISI 1040 525 149

AISI 420 655 195

AISI 4140 1020 302

AISI 316 550 149

2. Experimental Procedure The four different grades of steel – AISI 1040, 420, 4140, 316 Steels, respectively – were selected in order to compare the alteration of their mechanical properties after applying individually the three main nitriding methods. The specimens were carefully cut by an abrasive cutter, one of each specimen groups was reserved for metallographic examination before the procedures, then two groups of specimens were sent to Tamçelik A.Ş., İstanbul for salt bath and gas nitriding process. The other group of specimens was also sent to ERMIR A.Ş, Bursa, for plasma nitriding. After treatment each specimen was mounted, subsequently ground, polished and finally etched by Nital etching solution for AISI 1040 steel about 50-55 sec, for AISI 4140 steel about 40 sec. The pre-etching with Nital for AISI 316 steel about 3 minutes and for AISI 420 steel about 1 minute was applied; subsequently diluted aqua regia was used for the final etching, about 5 minutes for AISI 316 steel and 2 minutes for AISI 420 steel. The microstructure was observed by optical microscope (Nikon Eclipse LV150) before and after each nitriding process. The hardness of each specimen was measured before and after treatment by JFE Advantech Sonohard SH-21. The coating thickness was detected by ElektroPhysik Minitest 1100. The differences on the mechanical properties depending on the type of the steel used in procedure and the procedure applied were studied.

3. Results and Discussion 3.1. Microstructural Analysis The optical microscope micrographs of steels used before treatment and after plasma, gas and salt-bath nitriding processes in this study are shown in Fig. 1 and Fig. 2, 3, 4, respectively.

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 1. Optical microscope micrographs of AISI 316 (a,b), 420 (c,d),1040 (e,f) and 4140 (g,h) steels before coating

applications with different magnifications (a-d, x20 and e-h, x50 ).

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 2. Optical microscope micrographs of AISI 316 (a,b), 420 (c,d),1040 (e,f) and 4140 (g,h) steels after plasma

nitriding process with different magnifications. (a-d, x20 and e-h, x50 ).

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(a)

(b)

(c)

(d)

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(f)

(g)

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Fig. 3. Optical microscope micrographs of AISI 316 (a,b), 420 (c,d),1040 (e,f) and 4140 (g,h) steels after gas

nitriding process with different magnifications. (a-d, x20 and e-h, x50 ).

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Fig4. Optical microscope micrographs of AISI 316 (a,b), 420 (c,d),1040 (e,f) and 4140 (g,h) steels after salt-bath nitriding process with different magnifications. (a-d, x20 and e-h, x50 ).

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3.2. Mechanical Testing The mechanical properties of steels after each nitriding processes and their coating thicknesses are listed in Table 3.

Table 3. Comparison of mechanical properties and coating thickness of AISI 1040, 420, 4140 and 316 steels after

plasma, gas and salt-bath nitriding process.

Hardness (HRV) Tensile Strength (MPa)*** Coating thickness (μm)

Steel Plasma Nitriding

Gas Nitriding

Salt-bath nitriding

Plasma Nitriding

Gas Nitriding

Salt-bath

nitriding

Plasma Nitriding

Gas Nitriding

Salt-bath

nitriding

AISI 1040

245 (233HB**)

402 (380HB**)

524 (491HB**)

803 1311 1694 5.5 8.6 7.6

AISI 420

661 (621HB**)

294 (278HB**)

292 (276HB**)

2142 959 952 7.6 4.3 2.3

AISI 4140

515 (483HB**)

425 (401HB**)

489 (460HB**)

1666 1383 1587 10.4 9.3 8.6

AISI 316*

310 (294HB**)

296 (280HB**)

206 (196HB**)

1014 966 676 - - -

* As AISI 316 steel is non-magnetic, it was impossible to measure the coating thickness by this method and this instrument, so just the SEM results could be obtained and were used. ** Brinell hardness is converted from Rockwell C hardness [8]. *** Tensile strength is converted from Brinell hardness [9].

In this study, we examined the mechanical properties data obtained after each nitriding processes on four different grade steel substrates. In this context, the highest mechanical values were obtained for plasma nitriding process by coating on AISI 420 steels, for gas nitriding process by coating on AISI 4140 and finally for salt-bath nitriding process by coating on AISI 1040. Amongst the nitriding processes, the plasma nitriding process seems to be the most effective with an optimum coating thickness of 7.6 μm.

4. Conclusion

As a result of this work on the comparison of mechanical properties of AISI 1040, 420, 316 and 4140 steels after plasma, gas and salt-bath nitriding processes, it can be said that the thickness of nitrided layers varies, but the thickness and the hardness of the specimens were not correlated due to the differences on the formation mechanisms of coating layers.

5. References [1] Nishimoto, A., Akamatsu, K., Effect of Pre-Deforming on Plasma Nitriding Response of 304 Stainless Steel,

Materials Science Forum, Vols. 654-656, 1811-1814, 2010 [2] Li , C. X., Bell, T., A comparative study of low temperature plasma nitriding, carburising and nitro- carburising of AISI 410 martensitic stainless steel, Materials Science and Technology, Vol. 23, 3, 355, 2007 [3] Nouveau, C., Steyer, P., K. R. M., Lagadrillere, D., Plasma nitriding of 90CrMoV8 tool steel for the enhancement of hardness andvcorrosion resistance, Surface & Coatings Technology 205, 4514- 4520, 2011 [4] Wang, B., Hou, Z., Wang, W., Zhao, B., Investigation of gas nitriding on wear and corrosion behavior of 40Cr steel, Advanced Materials Research Vols. 311-313, 674-678, 2011 [5] Pinedo, C. E., Monteiro, W. A., On the kinetics of plasma nitriding a martensitic stainless steel type AISI 420, Surface and Coatings Technology 179, 119–123, 2004 [6] Metallography and Microstructures, ASM Handbook, Volume 9, 5

th printing,1992.

[7] Metals Handbook Desk Edition, ASM International, ed. Joseph R. Davis, 2nd

ed., 1998. [8] Mechanical Testing, ASM Metals Handbook, Volume 8. [9] Materials Science and Engineering, An Introduction, William D. Callister, Seventh ed., 2007.