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Mechanical Properties of a Ni–Cr–Mo Steel Subjected to Room Temperature Carburizing Using Surface Mechano-Chemical Carburizing Treatment (SMCT)

  • Jogindra Nath Sahu
  • C. Sasikumar
Technical Paper
  • 44 Downloads

Abstract

Surface mechano-chemical carburizing treatment (SMCT) is a modified version of surface mechanical attrition treatment and it is one of the cutting-edge technologies for producing hard nano-crystalline surface in metallic materials. In the present study, a case carburized surface layer is achieved in 1.75 Ni–Cr–Mo steel at room temperature using SMCT. Activated charcoal powder is continuously fed during the process so as to achieve the carbon diffusion into the surface layer. The SMCT process has been carried out for different periods say 15, 30, 45 and 60 min respectively. The microstructure and surface chemical composition is investigated by using TEM and XRF analysis. The mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), fracture toughness and surface hardness of SMCT samples have been investigated using universal testing machine, Plain strain fracture toughness test and Microvickers hardness test respectively. The surface carbon content has been found to increase linearly and grain size reduces continuously with processing time. A 60 min SMCT samples reveal 0.8% C and about 10 nm grains over the surface. The SMCT samples show significant improvement in mechanical properties. The surface hardness increases from 180 HV0.1 to ~ 878 HV0.1 by 60 min of treatment. About 55% increment in the YS and 30% increment in UTS is achieved by 60 min of SMCT. It is also interesting to note that the fracture toughness of the samples enhances from 24 to 47 MPa \( \sqrt m \) after 60 min of SMCT.

Keywords

Surface nano crystallization Mechanical attrition force Fracture toughness Activated charcoal and surface mechano chemical carburizing treatment 

1 Introduction

Mechanical alloying (MA) is wildly known powder processing techniques introduced by Benjamin at International Nickel Company (INCO) to achieve oxide dispersion strengthened (ODS) superalloys for gas turbine applications [1]. This process has opened new pathways for synthesizing advanced materials consisting of equilibrium or non-equilibrium phases in particular at room temperature. Researchers have synthesized supersaturated solid solutions, intermetallic compounds, meta-stable phases, quasi-crystalline and amorphous phases [2]. The process has received a new outlook when reaction milling is reported i.e. chemical reactions are induced by mechanical milling [3]. Jangg et al. [4] have pioneered in reaction milling process. They have achieved fine dispersion of oxides/nitrides/borides or carbides by reaction milling in a controlled atmosphere consisting of solidus/gaseous precursors. Researchers have synthesized insitu aluminium carbide (Al4C3) dispersed in aluminium matrix by milling Al with lamp black/graphite powder [5, 6]. The research works of mechanical alloying/milling have received further attention while mechano-chemical processing (MCP) is reported. MCP claims solid state reactions and phase transformations at room temperature. In particular the carbide formation and dispersion is reported elsewhere [7]. Interestingly, in situ formation of Al4C3, Co3C, MoC, TiC, Ti2AlC, Ni3C, Cr3C2, Cr23C6, VC, WC, Fe3C, Fe2C, Fe5C2, Fe7C3, etc are achieved by mechanochemical processing [8, 9, 10]. It is found that few types of carbide form directly by milling while others form after subsequent annealing. Basset and coworkers [11] have reported that Fe3C forms directly after 5 h planetary ball milling of Fe and C powders.

Though mechanical alloying/mechano-chemical processing using ball mills has notable advantage in synthesizing advanced materials, it has its own limitations for industrial bulk scale production. A novel process similar to mechanical milling is recently introduced by the researchers to achieve surface nanocystallization in metallic materials. The process is termed as surface mechanical attrition treatment (SMAT) and in this process, fast moving balls are made to collide on the surface of metallic materials similar to mechanical milling [12]. However, in contrast to planetary ball milling, the balls are not confined to small grinding bowls. There by, higher impact energy can be achieved in the collision incident. Lu and Lu [13] are known to be pioneers in this field and their team have investigated the microstructural changes and surface mechanical properties of materials subjected to SMAT. Researchers have demonstrated that it is possible to produce 10 nm or finer crystals by SMAT for the duration of 30–60 min [14]. Surprisingly, no reports are found on mechano-chemical processing induced by SMAT. In the present work, an attempt is made to combine the advantages of mechano-chemical processing (MCP) and SMAT. Synthesis of iron-carbide (Fe3C) over a case carburizing grade steel is considered for the present study. Since carburized surface is achieved by this process, the process is termed as surface mechano-chemical carburizing treatment (SMCT). A low temperature nitriding/boriding is achieved by few researchers [15, 16]. Few papers claim room temperature metallic coatings [17]. However, room temperature mechano-chemical carburizing is not reported elsewhere. In our present work, the carburized surface is produced in a single step at room temperature. The microstructure and mechanical properties of the steel case carburized by SMCT is investigated in detail. The mechanical properties of these steel samples have been compared with conventional case carburized steels.

2 Experimental

2.1 Materials

The material used in present study was a commercial grade 1.75 Ni–Cr–Mo case hardening steel used in the manufacturing gears for railway traction purpose. About 250 × 35 × 20 mm of samples were prepared for the present investigation. The chemical compositions of the samples are shown in the Table 1.
Table 1

Chemical composition of Ni–Cr–Mo steel used in the present investigation

Material

Elements present (in weight percent)

C

Si

Mn

Cr

Mo

S

Ni

P

Others

Fe

Ni–Cr–Mo steel

0.17

0.25

0.76

0.97

0.13

0.01

1.75

0.002

0.38

Balance

2.2 Surface Mechano-Chemical Carburization Treatment (SMCT)

The Ni–Cr–Mo steel strips were subjected to case carburizing at room temperature using surface mechano-chemical carburizing treatment (SMCT). The schematic diagram of the experimental setup used for surface mechano-chemical carburizing treatment is shown Fig. 1. The equipment consisted of a peening gun, working chamber and shot storage tank. The peening gun essentially consisted of a rotating wheel, which was used to throw the steel shots at different (20–50 m/s) velocities. The working chamber consisted of a fixer and satellite table. By using this satellite table, it was possible to rotate the sample, so that uniform processing could be achieved. The Ni–Cr–Mo steel samples were fixed into the chamber and subjected to collision with shots. A martensitic grade stainless steel shots were used for surface collision. During the process, the activated charcoal in slurry form was fed continuously over the samples to achieve surface mechano-chemical carburizing. No external heating was provided and the process was continued for 15 min. The experiments were repeated with different samples for different time periods, i.e. for 30, 45 and 60 min respectively. Few experiments were carried out without carbon for 15 min to study the difference between SMAT and SMCT.
Fig. 1

Experimental setup used for surface mechano-chemical carburization treatment

2.3 Characterization

The samples subjected to SMCT were carefully prepared to investigate the surface chemical composition, microstructure and mechanical properties. The variation of the surface carbon content by SMCT was investigated by using a Fischer XRF (model XDAL). The equipment was properly calibrated with standard samples prior to the measurements. A JEOL (JEM 2100F) transmission electron microscope (TEM) was used to study the surface microstructures before and after treatment. A 1000 kN Instron universal testing machine was used to investigate the mechanical properties such as YS, UTS and fracture toughness of the material. The tensile samples were made as per ASTM A370. A sheet type specimen having 12.5 mm width, 12.5 mm thickness and 50 mm gauge length was prepared for tensile test and strain rate was fixed for all experiments. The fracture strengths of the materials were investigated using bending specimen as per ASTM E399 plain strain fracture toughness method. A Buehler make micro Vickers hardness tester was used to determine the surface hardness of the Ni–Cr–Mo steel after room temperature case carburization using SMCT.

3 Results and Discussions

3.1 Surface Chemical Composition

The XRF surface chemical compositions of the samples at different periods of surface mechano-chemical carburizing treatment (SMCT) are shown in Table 2
Table 2

XRF results of samples subjected to different periods of SMCT

Time of SMCT

Elements present (in weight percent)

Minutes

C

Cr

Ni

Mo

Fe

0

0.2

0.77

1.81

0.24

Bal.

15

0.4

0.63

1.27

0.30

Bal.

30

0.5

0.85

1.34

0.20

Bal.

45

0.6

0.80

1.78

0.22

Bal.

60

0.8

0.71

1.69

0.21

Bal.

As shown in the Table 2, the surface carbon content varies significantly by SMCT over the entire surface. A linear increment in surface carbon content is observed. A minor variation in Cr, Ni and Mo content is also noticed. However, these variations are localized and found insignificant. The variations of surface Cr, Ni and Mo can be attributed to the contaminations caused by martensitic grade stainless steel shots used for SMCT. Since Cr and Mo are strong carbide forming elements, they are found to react with surface carbon at few locations. This incident has altered Ni content also however, their contribution is found to be localized and insignificant. The aforementioned results clearly reveal that carbon is mechanically alloyed over the surface of the steel.

3.2 TEM Analysis

The TEM images of the samples before and after SMCT for 30 min are shown in Fig. 2. The selected area electron diffraction (SAED) pattern is also shown as inset. The microstructures clearly reveal predominantly ferrite phases before SMCT and formation of carbides after surface mechano-chemical treatment. The SAED pattern shows the presence of α-Fe in untreated samples and Fe3C phases in treated samples. The aforementioned results confirm the mechano-chemical reaction of Fe and C during the process. The grain size is also found to be refined considerably by SMCT. About 10 nm grains are achieved in 30 min SMCT samples. This can be attributed to the severe plastic deformation of the surface.
Fig. 2

TEM images of samples a before SMCT and b after 30 min of SMCT revealing the formation of carbides. The inset figures shows SAED pattern of α-Fe and carbide phases respectively

3.3 Variation of Surface Hardness

The variations of the surface hardness of samples subjected to room temperature case carburization using SMCT for 0, 15, 30, 45 and 60 min are shown in Fig. 3. The hardness of SMAT 15 i.e. samples treated without carbon for 15 min has also been incorporated into the figure to differentiate the results of SMAT and SMCT.
Fig. 3

Variation in the surface hardness of Ni–Cr–Mo steel subjected to SMCT for 0, 15, 30, 45 and 60 min respectively. The hardness of SMAT sample for 15 min is also provided for comparison

The hardness values are obtained over the treated surface at different locations. The untreated samples show an average hardness of 180 HV0.1. The SMCT samples show considerable increment in the surface hardness. The average values of surface hardness obtained by SMCT are listed in Table 3.
Table 3

Average surface hardness of samples subjected to SMCT for different time periods

Process time (minutes)

Average surface hardness achieved (HV0.1)

0

180

15

408 (SMAT 15-332)

30

715

45

827

60

878

The surface hardness has been found to be increasing exponentially with process time. A maximum of 878 HV0.1 is achieved after 60 min of SMCT. The increment in surface hardness is achieved by surface nanocystallization as well as carbide formation due to mechano-chemical reaction. The increment in surface hardness of steel subjected to surface nanocystallization is reported elsewhere [18, 19]. However, they can achieve a maximum hardness of 500 HV0.1 after 60 min of SMAT. The maximum hardness achieved in the present study is (878 HV0.1), equivalent to the one achieved by conventional gas carburizing (850–950 HV0.1) [20]. This is a remarkable outcome of the surface mechano-chemical carburizing treatment without exposing the material to a higher temperature (900–1050 °C) for a longer period (8–16 h).

3.4 Tensile Behavior of SMCT Samples

The typical tensile behaviors of the samples subjected to SMCT for 0, 30 and 60 min are shown Fig. 4. As shown, the YS and UTS of the samples increases significantly by SMCT. The YS of the Ni–Cr–Mo steel in untreated condition is 430 MPa and it found to increase to 661 and 669 MPa after 30 and 60 min of SMCT respectively. About 55% increment in the YS is achieved by 60 min of SMCT. This significant change can be attributed to the surface nano-crystallization and carbide formation by SMCT as revealed in TEM analysis. The decrement of grain size can lead to grain boundary strengthening while the carbides can lead to precipitation hardening.
Fig. 4

Engineering stress versus strain data of SMCT samples

It is reported that the surface grains require lesser stress for the deformation in comparison to the interior grains [21].Thus the overall strength of materials can be improved by strengthening the surface grains. The overall strength (σ) of any material is given by
$$ \sigma = V_{\text{f}} \sigma_{\text{s}} + (1 - V_{\text{f}} )\sigma_{\text{i}} ,\quad (1)\;[21] $$
where σs and σi are the stresses necessary to deform surface and interior grains, respectively, and Vf is the volume fraction of surface grains. In the present study the surface grains are strengthened by carbide formation due to surface mechano-chemical reaction. The ultimate tensile strength of steel is 549, 702 and 714 MPa after 0, 30 and 60 min of SMCT respectively. The UTS increases to about 30% by 60 min of SMCT. It is also interesting to note that the material withstand a larger strain even after necking. This indicates that the SMCT steel have better fracture toughness in comparison to the untreated one.

3.5 Fracture Toughness

The fracture toughness of the samples subjected to SMCT is obtained by using a bending test. The samples prepared as per ASTM E399 are depicted in the inset of Fig. 5. The energy required to open a preexisting crack is evaluated in untreated and treated samples. The fracture toughness of the material is calculated using the following equation.
$$ {\text{K}}_{\text{Q}} \equiv {\text{Q}}_{\text{C}} \cdot \sigma_{\text{Q}} \cdot \sqrt {\varPi a} \quad (2)\;[22] $$
where QC is configuration correction factor for a specimen geometry of relative crack depth, a/w, where ‘a’ is crack length in meter, ‘w’ is represented to width of samples in meter, σQ ≡ PQ/(B), where, PQ is the load corresponding to the specified geometry (a/w), B is the specimen thickness.
Fig. 5

Fracture toughness trend of SMCT and untreated samples

σQ is the stress calculated at PQ. The energy required for opening a pre-existing crack in untreated and 15, 30, 45 and 60 min SMCT samples are shown in Fig. 5.

As shown in Fig. 5, the energy required to open a new surface increases significantly for the samples subjected to SMCT. The results of fracture toughness evaluated by plane strain fracture toughness test are summarized in Table 4.
Table 4

Fracture toughness of room temperature case carburized steel using SMCT for different time periods

S. no

SMCT process time

Fracture toughness ( MPa \( \surd m \))

1

SMCT 0

24

2

SMCT 15

26

3

SMCT 30

39

4

SMCT 45

45

5

SMCT 60

47

The fracture toughness of the steel increases exponentially with process time. About 93% increase in fracture toughness is achieved by 60 min of SMCT. Similar results are reported by other researchers on nanocrystalline materials [23, 24, 25]. Liu et al. [24] have carried out detailed investigation on fracture toughness of nanocrystalline materials. Their team have identified that the processes of crack blunting and propagation are controlled by inter-grain sliding in the vicinities of the crack tips. The inter-grain sliding creates dislocations at grain boundaries, which is responsible for crack propagation. Their group has demonstrated that the fracture toughness can be doubled in nanocrystalline materials. Further, toughening effect is also sensitive to ultrafine or nano-crystals [24]. The fracture toughness of the same material subjected to thermo-chemical carburization and subsequent oil quenching is reported to be 36 MPa \( \surd m \) [26]. Thus the steel samples carburized by SMCT show improved fracture toughness in comparison to a conventionally treated samples.

The aforementioned results clearly reveal that it is possible to case carburize a steel at room temperature by using SMCT. Further, it is found that the process time (~ 1 h) can be significantly reduced in comparison to a conventional thermo-chemical process (8–16 h). The mechanical properties such as the surface hardness, YS, UTS and fracture toughness can be improved by SMCT.

4 Conclusions

Case carburizing of Ni–Cr–Mo Steel was successfully developed at room temperature by surface mechano-chemical carburizing treatment. These room temperature case carburized samples showed significant improvement in mechanical properties such as yield strength, UTS, surface hardness and fracture toughness. The typical outcomes achieved out of this study are listed below.
  1. 1.

    The surface carbon content of the samples was found to be increasing exponentially with process time. The untreated sample showed a surface carbon of 0.2% and by SMCT, the surface carbon content had reached a maximum of about 0.8 wt%.

     
  2. 2.

    The SMCT process had resulted in the formation of Fe3C and reduced the grain size to 10 nm and finer.

     
  3. 3.

    The formation of the hard Fe3C layer and nanocrystalline surface had enhanced the surface hardness of 60 min room temperature case carburized layer by SMCT to about 878 from 180 HV0.1.

     
  4. 4.

    A significant increase in yield strength, UTS and toughness of the material was observed. The yield strength of untreated material was 460 MPa, while it reached above 660 MPa by SMCT. This considerable change could be attributed to the strengthening of the surface grains by carbides as well as grain boundary strengthening.

     

It was interesting to note that the fracture toughness of the materials case carburized by SMCT showed a significant enhancement. The fracture toughness evaluated using a bending test was 47 MPa \( \surd m \) after 60 min of SMCT while it was 24 MPa \( \surd m \) for the untreated sample. This could be due to the nature of crack propagation in a Nano crystalline surface layer compared to its bulk counterpart.

5 Acknowledgement

The authors are thankful to the management BHEL, Bhopal for the assistance in SMAT process. The authors are thankful to Director MANIT, Bhopal for providing characterization facility and other technical assistance. The authors are thankful to the engineers and section in-charges of shot peening, XRF sections of BHEL, Bhopal. The authors are also thankful to the professor-in-charge IIT Kharagpur for TEM analysis.

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Copyright information

© The Indian Institute of Metals - IIM 2017

Authors and Affiliations

  1. 1.Department of Materials and Metallurgical EngineeringMANITBhopalIndia

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