Corrosion Behavior and Characterization of Plasma Nitrided and Borided AISI M2 Steel
Ibrahim Gunesa*,Muzaffer Erdoganb, Atila Gürhan Çelikc
aDepartment of Metallurgical and Materials Engineering, Afyon Kocatepe University, 03200, Afyonkarahisar, Turkey bDepartment of Automotive Engineering, Faculty of Technology,
Afyon Kocatepe University, 03200, Afyonkarahisar, Turkey
cNational Boron Research Instıtute, Dumlupınar Boulevard, 06520, Ankara, Turkey
Received: July 4, 2013; Revised: March 27, 2014
This study investigates the corrosion behaviors of plasma nitrided (PN) and borided AISI M2 steels. The PN process was carried out in a dc-plasma system at a temperature of 923K for 6 h in a gas mixture of 80%N2-20%H2 under a constant pressure of 5 mbar. The boriding process was carried out in Ekabor-II powder at a temperature of 1223K for 6 h. X-ray diffraction analysis on the surface of the PN and borided steels revealed the presence of FeB, Fe2B, CrB, MoB, WB, FeN, Fe2N, Fe3N
and Fe4N compounds. Corrosion surfaces of the samples were analyzed using a SEM microscopy and
X-ray energy dispersive spectroscopy EDS. The corrosion resistance of PN and borided AISI M2 steel is higher compared with that of untreated AISI M2 steel. Corrosion resistances of the PN and borided AISI M2 steel increased the 4-6- fold.
Keywords: plasma nitriding, boriding, AISI M2, micro-hardness, corrosion
Plasma nitriding and boriding are a surface hardening and advanced surface modification technology which has recently become industrially important1. It is used for
increasing wear resistance, fatigue strength, surface hardness and corrosion resistance of industrial components for a wide variety of applications1-3. Nitriding of steel results
in formation of a compound layer and nitrogen diffusion zone beneath the compound layer. The diffusion layer is composed of nitrogen saturated ferrite together with dispersed precipitates of iron and alloy element nitrides, but the compound layer is mainly a combination of iron nitrides4.
Boriding is a diffusion surface treatment, which is defined as the enrichment of the surface of a workpiece with boron by means of thermo-chemical treatment5-7.Thermal
diffusion treatments of boron compounds used to form iron borides typically require process temperatures of 973 and 1273K. The process can be carried out in laser, solid, liquid, gaseous or plasma media. Boron atoms, because of their relatively small size (atomic radius 90 pm) and very mobile nature can diffuse into substrate material (atomic radius 126 pm). When iron, the most current substrate metal, boron atoms dissolve in iron interstitially, but can also react with it to form boride layer. Boron atoms due to their relatively small size and very mobile nature can diffuse easily into ferrous alloys forming FeB and Fe2B intermetallic, nonoxide, ceramic borides.The diffusion of B into steel results in the formation of iron borides (FeB and Fe2B) and the thickness of the boride layer is determined by the temperature and time of the treatment8-14.
Boriding and plasma nitriding are used in numerous applications in industries such as the manufacture of
machine parts for plastics and food processing, packaging and tooling, as well as pumps and hydraulic machine parts, crankshafts, rolls and heavy gears, motor and car construction, cold and hot working dies and cutting tools. The corrosion behavior of plasma nitrided and borided steels has been evaluated by a number of investigators15-21.
However, there is no information about the corrosion behaviors of plasma nitrided and borided AISI M2 steel. The main objective of this study was to investigate the corrosion behaviors of plasma nitrided and borided AISI M2 steel. Structural and corrosion properties were investigated using optical microscopy, microhardness tests, XRD, SEM and EDS analysis.
2. Experimental Procedures
Plasma nitriding, boriding and
Table 1 gives the composition of untreated AISI M2 steel. The test samples were cut into Ø25x8mm dimensions and ground up to 1000G and polished using diamond solution. The PN process was carried out in a dc plasma system at a temperature of 923K for 6 h in a gas mixture
of 80%N2-20%H2 under a constant pressure of 5 mbar.
Boriding heat treatment was carried out by using a solid boriding method with commercial Ekabor-II powders. Boriding heat treatment was performed in an electrical resistance furnace under atmospheric pressure at 1223K for 6 h followed by cooling in air.
The microstructures of the polished and etched cross-sections of the samples were observed under an Olympus
BX-60 optical microscope. The presence of nitrides and borides formed in the coating layer was detected by means of the X-ray diffraction equipment (Shimadzu XRD 6000) using Cu Kα radiation. The diffusion zone on the nitrided sample
and the thickness of borides were measured by means of a digital thickness measuring instrument attached to an optical microscope (Olympus BX60). Micro-hardness measurements were performed from the surface to the interior along a line to see variations in the hardness of the nitride and boride layer, transition zone and matrix, respectively. The micro-hardness of the boride layers was measured at 8 different locations at the same distance from the surface by means of a Shimadzu HMV-2 Vickers indenter with a load of 50 g and the average value was taken as the hardness.
The acid solution used was 4% M HCI. The aforementioned cylindrical borided specimens and non-borided specimens were weighted before immersion, with
an accuracy of 0.01 mg. At specific time intervals the specimens were withdrawn from the solutions and weighted without any additional treatment. Thus, the weight loss in relation to the initially exposed surface was continuously recorded. The immersion tests were repeated 10 times and mean values were used for the acquisition of weight loss curves. Before and after each corrosion test, each sample was cleaned with alcohol.
3. Results and Discussion
Characterization of plasma nitriding and
The cross-sections of the optical micrographs of the plasma nitrided and borided M2 steel are given in Figure 1. A typical optical photograph of the cross-sectional microstructure of the plasma nitrided M2 sample is given in Figure 1a. This photograph shows the formation of the
Figure 1. Cross-section of plasma nitrided and borided AISI M2 steel. a) PN, b) borided.
Figure 2. X-ray diffraction patterns of plasma nitrided and borided AISI M2 steel a) PN, b) borided. Table 1. Chemical composition of test materials (wt.%).
Steels C Cr Mo W V Ni Mn Fe
Figure 3. Variation of hardness depth in plasma nitrided and borided AISI M2 steel.
Figure 4. SEM and EDS analysis of the unborided AISI M2 steel a) before corrosion, b) after corrosion, c) EDS analysis.
diffusion zone on the sample. However the formation of a continuous compound (white) layer on top of the nitrided layer of the sample is not visible in Figure 1a. After etching with 3% nital reagent, the nitrided (diffusion) layer appears as a dark zone on the sample. As can be seen in Figure 1b,
the boride layer was formed on the M2 steel borided by a solid boriding method. Depending on the process time, temperature and chemical composition of substrates, the depth of the boride layer was found to be 55 µm.
The X-ray diffraction patterns of the nitrided and borided AISI M2 steel are given in Figure 2. XRD results showed that nitrided AISI M2 steel contained FeN, Fe2N, Fe3N, Fe4N
and M6C phases (Figure 2a). XRD results showed that the boride layers formed on the AISI M2 steel contained FeB, Fe2B, CrB, MoB and WB phases in Figure 2b. The corrosion resistances of these nitride and boride layers are to a large extent known by the help of these phases.
Micro-hardness measurements were carried out on the cross-sections from the surface to the interior along a line in Figure 3. The hardness of the nitrided AISI M2 steel varied between 995 and 1286 HV0,05. The hardness of the
Figure 5. SEM and EDS analysis of the plasma nitrided AISI M2 steel a) before corrosion, b) after corrosion, c) EDS analysis.
Figure 4 shows the SEM and EDS (Figure 4c) analysis of the untreated AISI M2 steel before (Figure 4a) and after (Figure 4b) the corrosion test. Pores and pits were detected on the surface of the specimen after the corrosion test. In addition, oxides were observed on the surface of the specimen after the corrosion test. Figures 5a and 6a show the SEM microstructures of the specimens before the corrosion process while Figures 6b and 7b show the SEM microstructures of the specimens after the corrosion process. Oxides were observed on the surface of the specimen after the corrosion test. Oxide peak intensities formed on the surface of the specimens were found to decrease after the plasma nitriding and boriding treatments (Figure 5c and Figure 6c). After the corrosion test, porosity and pits were observed to form on the nitrided specimen. The corrosion resistance of a nitrided steel usually depends on the characteristic features of coatings such as the number of porosities (Figure 6b). These porosities negatively affect the adhesion of coatings and significantly reduce the corrosion resistance. The number of these voids is associated with
the microstructure of the coating22-24. Figure 7 shows the
The lowest weight loss was observed in the borided AISI M2 specimen (93.21 mg.) at the temperature of 1223K for 6 h. While the weight loss of the untreated specimen was 561.82 mg., this value dropped to 93.21 mg. as a result of the boriding process for AISI M2 steel. The solubility of corrosion in the untreated
specimen increased 6 times. As shown in the corrosion graphic curves, the solubility of corrosion decreased with the plasma nitrided and borided treatment. In addition, it was observed that the decreased solubility of corrosion in the borided specimens led to a decrease in the amount of material loss. It was observed that the solubility
Figure 6. SEM and EDS analysis of the borided AISI M2 steel a) before corrosion, b) after corrosion, c) EDS analysis.
Figure 7. Surface roughness values of plasma nitrided and borided AISI M2 steel.
of corrosion of the borided specimens and that of the plasma nitrided specimens were 6 times and 4 times lower than that of the untreated specimen, respectively. This significant decrease in corrosion rate and increase in corrosion resistance might be attributed to the formation of a protective surface coating treatment. As result of the corrosion resistances of the AISI M2 steel increased with the plasma nitriding and boriding treatment.
The following conclusions may be derived from the present study.
• The multiphase nitride coating which thermo-chemically occurred on the AISI M2 steel was constituted by the FeN, Fe2N, Fe3N, Fe4N and M6C
phases while the boride coating was constituted by
the FeB, Fe2B, CrB, MoB and WB phases;
• ThesurfacehardnessofthenitridedAISIM2steelwas in the range of 995-1286 HV0,05, the surface hardness of the borided AISI M2 steel was in the range of 1378-1814 HV0,05 and the untreated AISI M2 steel substrate was 230 HV0,05;
• Thesurfaceroughnessvaluesoftheuntreated,plasma nitrided and borided AISI M2 steel increased after corrosion tests;
• Thecorrosionresistancesoftheplasmanitridedand borided AISI M2 steel increased 4-6- fold compared to that of the untreated AISI M2 steel;
• ThesuperiorpropertiesoftheAISIM2steelaswell as poor corrosion properties were improved by the plasma nitriding and boriding process.
1. Ahangarani S, Sabour AR, Mahboubi F and Shahrabi T. The influence of active screen plasma nitriding parameters on corrosion behavior of a low-alloy steel. Journal of Alloys and Compounds. 2009; 484:222-229. http://dx.doi.org/10.1016/j. jallcom.2009.03.161
2. Sinha AK. Boriding (Boronizing), ASM Handbook.Journal of Heat Treatment. 1991; 4:437-447.
3. Rihan RO. Electrochemical corrosion behavior of X52 and X60 steels in carbon dioxide containing saltwater solution. Materials Research. 2013; 16(1):227-236. http://dx.doi. org/10.1590/S1516-14392012005000170
4. Ebrahimi M, Heydarzadeh SM, Honarbakhsh RA and Mahboubi F. Effect of plasma nitriding temperature on the corrosion behavior of AISI 4140 steel before and after oxidation. Surface and Coatings Technology. 2010; 205:261-266. http://dx.doi.org/10.1016/j.surfcoat.2010.07.115
5. Gunes I. Kinetics of borided gear steels. Sadhana. 2013; 38:527-541. http://dx.doi.org/10.1007/s12046-013-0138-0
6. Matuschka AG. Boronizing. Philadelphia: Heyden and Son. Inc.; 1980. p. 11-31.
7. Bindal C and Ucisik AH. Characterization of borides formed on impurity-controlled chromium-based low alloy steels. Surface and Coatings Technology. 1999; 122:208-213. http://dx.doi. org/10.1016/S0257-8972(99)00294-7
8. Gunes I and Taktak S. Surface characterization of pack and plasma paste boronized of 21NiCrMo2 steel. Journal of the Faculty of Engineering and Architecture of Gazi University. 2012; 27:99-108.
9. Ozdemir O, Omar MA, Usta M, Zeytin S, Bindal C and Ucisik AH. An investigation on boriding kinetics of AISI 316 stainless steel. Vacuum. 2008; 83:175-179. http://dx.doi.org/10.1016/j. vacuum.2008.03.026
10. Gunes I, Kayali Y and Ulu S. Investigation of surface properties and wear resistance of borided steels with different B4C mixtures. Indian Journal of Engineering Materials Science. 2012; 19:397-402.
11. Ozbek I, Sen S, Ipek M, Bindal C, Zeytin S and Ucisik AH. A mechanical aspect of borides formed on the AISI 440C stainless-steel. Vacuum. 2004; 73:643-648. http://dx.doi. org/10.1016/j.vacuum.2003.12.083
12. Ulker S, Gunes I and Taktak S. Investigation of tribological behavior of plasma paste boronized of AISI 8620, 52100 and 440C Steels. Indian Journal of Engineering Materials Science. 2011; 18:370-376.
13. Uslu I, Comert H, Ipek M, Ozdemir O and Bindal C. Evaluation of borides formed on AISI P20 steel. Materials and Design. 2007; 28:55-61. http://dx.doi.org/10.1016/j. matdes.2005.06.013
14. Gunes I. Wear behaviour of plasma paste boronized of AISI 8620 steel with borax And B2O3 paste mixtures. Journal of Materials Science &Technology. 2013; 29:662-668. http:// dx.doi.org/10.1016/j.jmst.2013.04.005
15. Xi Y, Liu D and Han D. Improvement of erosion and erosion-corrosion resistance of AISI420 stainless steel by low temperature plasma nitriding. Applied Surface Science. 2008; 254(18):5953-5958. http://dx.doi.org/10.1016/j. apsusc.2008.03.189
16. Wen DC. Plasma nitriding of plastic mold steel to increase wear and corrosion properties. Surface and Coatings Technology. 2009; 204(4):511-519. http://dx.doi.org/10.1016/j. surfcoat.2009.08.023
17. Basu A, Majumdar JD, Alphonsa J, Mukherjee S and Manna I. Corrosion resistance ımprovement of high carbon low alloy steel by plasma nitriding.Materials Letters. 2008; 62(17-18):3117-3120. http://dx.doi.org/10.1016/j.matlet.2008.02.001 18. Gontijo LC, Machado R, Kuri SE, Casteletti LC and
Nascente PAP. Corrosion resistance of the layers formed on the surface of plasma-nitrided AISI 304L. Thin Solid Films. 2006; 515(3):1093-1096. http://dx.doi.org/10.1016/j. tsf.2006.07.075
19. Campos I, Palomar M, Amador A, Ganem R and Martinez J. Evaluation of the corrosion resistance of iron boride coatings obtained by paste boriding process. Surface and Coatings Technology. 2006; 201(6):2438-2442. http://dx.doi. org/10.1016/j.surfcoat.2006.04.017
20. George K, Kariofillis G, Kiourtsidis E and Dimitrios NT. Corrosion behavior of borided AISI H13 hot work steel,Surface and Coatings Technology. 2006; 201(1-2):19-24. http://dx.doi. org/10.1016/j.surfcoat.2005.10.025
22. Liu C, Lin G, Yang D and Qi M. In vitro corrosion behavior of multilayered Ti/TiN coating on biomedical AISI 316L stainless steel. Surface and Coatings Technology. 2006; 200:4011-4016. http://dx.doi.org/10.1016/j.surfcoat.2004.12.015
23. Campos I, Palomar-Pardave M, Amador A, Velazquez CV and Hadad J. Corrosion behavior of boride layers evaluated by the
EIS technique. Applied Surface Science. 2007; 253:9061-9066. http://dx.doi.org/10.1016/j.apsusc.2007.05.016