Analysis of tempering temperature and vegetable oil quenchant viscosity effect on mechanical properties of 42CrMo4 steel

Abstract

Environment is the main concern nowadays to protect the soil. Heat treatment processes used to enhance the properties of steel should not affect the environment badly. In this study, eco-friendly oil-quench specimens of 42CrMo4 steel are subjected to different tempering temperatures for about 4 hours and tested. Hardness, room temperature impact toughness, and microstructure of specimens were determined. Results showed that as the tempering temperature increased, there was an increase in impact toughness with slight decrease in the hardness. Blend oil-quench specimen, tempered at 450°C and 500°C, shows reasonably good hardness (40 and 39 Rc, respectively) and impact toughness (35 and 40 J, respectively). Response surface approach for combined optimization has given the optimized value for tempering temperature and oil viscosity as 453°C and 34.7 centistokes, respectively. Confirmation test results have shown that the regression equation developed is the best fit with percentage error less than 3%.

Keywords:

• microstructure
• heat treatment
• Pinnay oil
• Karanja seed oil
• tempering

1. Introduction

Automobile driving parts such as crankshafts, front axles, axle journals, and steering components are typically made of low-alloy steels like 42CrMo4 (AISI 4140). 42CrMo4 is often used when higher strength is needed. To enhance the mechanical properties of 42CrMo4 steel, austenitizing, quenching, and tempering can be used as heat treatment methods. They have a tempered martensite microstructure and are most frequently employed in this state. The martensitic steel can be made less brittle after tempering. The chromium and molybdenum present in 42CrMo4 make it considerably easier to heat treat, and this results in various strength and ductility combinations. More importantly, the addition of molybdenum prevents medium-carbon steels from becoming temper brittle (Bayrak et al., 2007; Chaouch et al., 2012; Zafra et al., 2018). The hardenability, strength, and wear resistance of the alloy steels of the 41xx series are improved by these trace levels of chromium and molybdenum. In these series, molybdenum between 0.15% and 0.25% combined with a little addition of chromium between 0.8% and 1.10% is the leading composition in 42CrMo4 grade (Meshref & Mazen, 2020).

Heat treatment of carbon steel is used to alter the mechanical characteristics of the material as per the requirement, which are typically ductility, hardness, yield, tensile strength, and impact resistance. During the heat treatment procedure, the electric conductivity, heat and corrosion resistance may be altered (Shaikh et al., 2016). To achieve the desired application-specific properties, numerous investigations are carried out on the materials, not only by quenching the materials in various media but also by using various techniques to improve the mechanical properties of the materials. The mechanical properties of steels are closely related to the microstructure produced by the heat treatments. The effect of cooling rate on the mechanical properties and microstructure of industrially treated steels is currently a topic of great interest. The microstructure of steels is significantly influenced by the cooling rate (Ismail et al., 2016; Sun et al., 2018). Quench hardening is used to promote the formation of supersaturated room temperature phases such as bainite or martensite while preventing the formation of stable ferrite or pearlite phases. The favourable microstructural changes occur intentionally during quenching or cooling from high temperature to room temperature, with cooling being more important than heating. The hardenability of a specific alloy, the thickness and shape of the concerned sections, and the rate of cooling expected to get the required microstructure all influence the choice of the quenching medium. Water, brine, oil, and synthetic solutions are the most common options available as quenchants. Water, while abundant and inexpensive, has the disadvantage of causing cracking or dimensional changes due to rapid cooling, while oil does not produce sufficient hardness due to the slow cooling process. Although polymer quenchants have the major disadvantage that their concentration is variable, they can provide a hardness between water and oil. For this reason, a suitable quenchant is needed that is both economically viable and can provide noticeable hardening. Therefore, biodegradable oil is gradually replacing conventional oil as it is safer, more environmentally friendly, and more cost effective (Salihu, 2013; Sreeja et al., 2016; Suleiman et al., 2018; Çalık et al., 2020).

Engineers have been continuously trying for many years to improve the qualities of alloy steels by making them stronger and tougher, especially on the impact of materials’ microstructures on mechanical properties. Design engineers typically concentrate on premature failure preventions by using stresses that are below the yield strength of a high-ductility material. Moreover, on newly produced high-strength and low-ductility alloys, however, the same procedure does not work correctly as there have been numerous reports of catastrophic failures. The toughness of a material was prioritized in design criteria for this very reason (Sakkaki et al., 2020).

Steels can be made more ductile, impact resistant, and less brittle by tempering, which is a popular technique. Carbon steels’ as-quenched toughness characteristics are frequently compromised by the rising carbon content and resulting rise in hardness and strength. Because of this, different tempering temperatures and durations can be used based on the desired mechanical properties, such as the appropriate hardness and impact toughness levels (Haiko et al., 2019). (Herbirowo et al., 2018).

Since the nineteenth century, petroleum-oil-based stocks have been the traditional oil-quench medium for hardening heat treatments. Mineral oils, on the other side, are not environmentally friendly since they are poisonous, non-biodegradable, and non-renewable. Vegetable oils, polymer quenchants, and nanofluids have all been created as environmentally acceptable alternatives to mineral oil. Sunflower, palm, soya bean, canola, and pongamia pinnata vegetable oils have thermophysical and wetting properties comparable to mineral oil, and even superior cooling efficacy (Ramesh & Prabhu, 2014; Totten et al., 1999). Because of global scarcity, using conventional fossil energy sources to create engine oils and additives comes with a number of challenges, including toxicity, non-biodegradability, and increasing costs. The world will face serious energy problems if alternative energy sources are not discovered because the rate at which mineral-based oil is consumed outpaces the rate at which its reserve is naturally created. In reaction to the rising demand for energy, lubricating oil manufacturers and consumers are implementing bio-based resources for the production of lubricants and additives, which has rekindled interest in the field. Unlike traditional mineral energy sources, bio-based energy is renewable, environmentally beneficial, and biodegradable (Owuna, 2020). Vegetable oil quenchant is extracted from vegetable sources, such as flowers, fruits, leaves, or seeds of various plants using mechanical methods or solvent extraction. Vegetable oils have long been utilized for hardening steels, and heat treaters have employed animal oils, vegetable oils, and a combination of both for quenching steels (Otero et al., 2012).

Plant oils, for example, sunflower, pongamia pinnata, palm, soybean, rapeseed, and coconut, are examples of vegetable oils. Bio oils are described as oils that are highly biodegradable and have low human and environmental toxicity. A component’s biodegradability refers to its ability to degrade in the presence of microorganisms. Study showed that vegetable oils are more biodegradable than mineral and other oils (Chandrakar & Suhane, 2014).

Because of the environmental and economic benefits, biodegradable oil is rapidly replacing the conventional oil as the media of quenching for steel’s heat treatment. However, from the literature review, it is noticed that not many reports are available regarding the effect of a few biodegradable oil quenchant on the mechanical properties of 42CrMo4 steel that is subjected to tempering treatment. In this study, an effort is made to fill this research gap by taking up the analysis regarding the effect of vegetable oil-quench viscosity along with tempering temperature on the hardness and impact energy of tempered 42CrMo4 steel. Pinnay oil, Karanja oil, and their blend are used as quenchants for 42CrMo4 steel. The microstructure and the mechanical properties are obtained for the different oil-quench viscosity and tempering temperature. Statistical analysis is used to get the regression equation to predict the hardness and impact toughness of the steel samples for the series of heat treatment parameters used in this study. Also, using the response surface methodology, optimized parameters of viscosity and tempering temperature are determined to get the good blend of hardness and impact energy of the steel. The novelty of this research work lies in the use of vegetable oil blend as a quenchant to balance the hardness and toughness properties.

2. Details of experiment

2.1. Material

42CrMo4 (AISI 4140) steel was purchased for the study. Composition of the steel was determined using the spectroscopy analysis. Table 1 displays the composition of both as-received steel and a standard 42CrMo4 steel.

Table 1. Chemical composition of 42CrMo4 steel

Element (wt. %)	C	Si	Mn	S	P	Cr	Mo	Fe
Standard range (Rajan & S, 2011)	0.35–0.45	0.1–0.35	0.5–0.8	0.05	0.05	0.90–1.5	0.2–0.4	Balance
As-received	0.43	0.29	0.80	0.016	0.018	0.91	0.26	Balance

2.2. Samples for Charpy impact test

The ASTM E-23 standard for Charpy impact specimens specifies a 42CrMo4 workpiece with dimensions of 55 × 10 × 10 mm and a V-notch machined over one of the upper dimensions. The V-notch has a base angle of 45°, a radius of 0.25 mm, and is 2 mm deep. Specimen preparation involves the use of a CNC lathe, a vertical milling centre, and wire EDM.

2.3. Microstructure study

Workpieces of 16 mm in diameter and 10 mm in length are machined for hardness test (ASTM E 18-02), and the same samples were used for the microstructure analysis. Samples were prepared using wire EDM. Emery sheets in the following grades are used to prepare the specimen surfaces for microstructure analysis: 400, 600, 800, 1000, 1500, and 2000. After the mirror polishing, they were further polished to ultra-fine level using an alumina paste on a polishing wheel. It is then etched with Nital before imaging to scanning electron microscope analysis and rinsed in water and dried. The microstructure imaging was conducted using SEM (Make: ZEISS, USA, Model: EVO MA 18).

2.4. Heat treatment

42CrMo4 steel samples received from the market are cut as per the test standards. Specimens were hardened by heating to 920°C for 2 hours to obtain complete austenite structure followed by quenching them with Pinnay seed oil, Karanja seed oil, and a combination of (blended) these two oils (60 volume percentage of Karanja oil and 40 volume percentage of Pinnay oil). After hardening, the specimens were immediately reheated to 350°C and 500°C, respectively, for tempering treatment, isothermally held for 4 hours, and then cooled in air. Using a Saybolt viscometer, oil viscosity was measured (Table 2). A Muffle electric furnace was used with a heatable capacity of up to 1000°C.

Table 2. Viscosity of quenchant oil

Quenchant oil	Kinematic viscosity (centistokes)
Karanja	30.77
Blended	32.63
Pinnay	41.12

2.5. Hardness and impact test

The hardness test procedure is followed as per the ASTM E-18 standard. Diamond cone indenter (load range of 150 kgf) of Rockwell hardness tester is used to ascertain the sample hardness. Charpy impact testing machine (Make: FIE, India, Model: IT-30 (STD)) is used to obtain the impact toughness of the samples.

3. Results and discussion

3.1. Microstructure study

Microstructures of the as-received samples of 42CrMo4 steel at different heat treatment conditions are shown in Figure 1. Scanning electron microscope is used to capture the specimen microstructure images.

Figure 1. As-received sample’s microstructure tempered at 350°C in different conditions: (a) Pinnay oil quench, (b) Blended oil quench, and (c) Karanja oil quench.

The as-received steel when quenched in Pinnay oil and tempered at 350°C for 4 h shows coarser grains as shown in Figure 1a. Feathery bainite is observed at this stage of treatment due to the slow cooling rate of thicker (high viscosity) oil. Martensite (coarse) is also observed as shown in Figure 1a. This bainite (feathery bainite) is also observed during hardening treatment. Moreover, black patches are thicker in size, indicating more carbides (Fe₃C) precipitation during tempering for such a larger (4 h) duration.

In Figure 1b, the tempered specimen in the blended oil at 350°C shows needle-type bainite, feathery bainite, and martensite. Due to the increase in fineness of phases, the hardness of the specimen is better than that obtained in Figure 1a. Black patches (carbide) are finer.

Figure 1c shows the Karanja oil quenched and tempered specimen. The presence of fine needle-type bainite, feathery bainite, and martensite structures is an indication of good hardness and toughness. Black patches (separated carbides) are still finer, contributing to hardness. Karanja oil is having low viscosity compared to the remaining (Pinnay and blended), showing faster heat removal, resulting in a fine microstructure.

Figure 2a shows the as-received Pinnay oil quenched and tempered condition (500°C) microstructure. Since the tempering temperature is quite high, more carbide precipitation is observed. Martensite structure changes into needle-type bainite structure.

Figure 2. As-received sample’s microstructure tempered at 500°C in different conditions: (a) Pinnay oil quench, (b) Blended oil quench, and (c) Karanja oil quench.

Figure 2b shows blended oil quenched and tempered (500°C) microstructure. It shows slightly lesser carbide precipitation as compared to Figure 2a condition.

Figure 2c shows Karanja oil quenched and tempered (500°C) specimen microstructure. Finer feathery bainite, needle bainite, and martensite are observed here. Carbide precipitation seems to be shallow compared to the previous two cases in the same category. If needle-type bainite observed has a finer colony and is finer in size, the impact toughness observed is very high without much compromise in hardness values.

From the microstructure (Figures 1 and 2), it is clear that tempering temperature has significant role on the microstructure of the quenched steel. As the tempering temperature increases, iron carbide content in the specimen increases due to the more diffusion of carbon from martensite (Rajan & S, 2011). Accordingly, the black regions (carbides) in the microstructure increase. At the same time, internal stress present in the specimen decreases due to the wide spacing of martensite or bainite needles. All these phenonomena improve the impact resistance of the material with the marginal sacrifice in hardness.

3.2. Hardness and impact test results

Hardness and impact toughness of 42CrMo4 steel samples at different tempering temperatures and quenchant oil viscosity are shown in Table 3 and Figure 3.

Figure 3. Effect of quenchant viscosity and temperature of tempering on hardness and impact toughness of 42CrMo4 steel.

Table 3. Hardness and impact toughness result for various conditions

Temperature of tempering (°C)	Quenchant oil	Quenchant viscosity (centistokes)	Hardness (Rc)	Impact toughness (J)
350	Karanja	30.77	41	13
400	Karanja	30.77	40	31
450	Karanja	30.77	39	33
500	Karanja	30.77	37	38
350	Pinnay	41.12	40	16
400	Pinnay	41.12	37	28
450	Pinnay	41.12	36	30
500	Pinnay	41.12	35	32
350	Blended	32.63	44	14
400	Blended	32.63	42	24
450	Blended	32.63	40	35
500	Blended	32.63	39	40

Table 3 shows the hardness and impact toughness values of the sample hardened and tempered at different temperatures in three different sample oils (Karanja, Pinnay, and blended). As the temperature of the tempering rises, the hardness declines whereas impact toughness increases. This behaviour remains same in all the oil samples. With the rise in the temperature of the tempering, the supersaturated martensite phase obtained on hardening is unstable with its existence. From the supersaturated solid solution martensite, the excess carbon atoms diffuse out so that the tetragonality of the martensitic cell (body-centred tetragonal (BCT)) decreases. In martensitic cell, the hardness depends upon the quantity of the carbon atoms trapped in the BCT structure. Since carbon diffuses out, the hardness of martensite decreases. The toughness (impact toughness) of martensite depends upon the tetragonality of the cell and is inversely proportional to the hardness trend (Avner, 1997). Higher the temperature, faster is the diffusion, and hence faster the rate of decrease in the height of the BCT cell. Hence, at high tempering temperature, hardness is less, whereas toughness is high. This trend is clearly observed from Figure 3 too.

3.3. Statistical analysis

A statistical analysis is done on the outcomes of impact toughness and hardness test. Analysis of variance test is used to examine the impact of each factor on these attributes.

Oil viscosity is the main influencing element in the analysis of impact toughness presented in Table 4, with a relative contribution of 49%, while temperature of tempering has a nearly 48% relative contribution on the variation of impact toughness for the values selected in the studied range.

Table 4. Results of ANOVA for impact toughness of the material

Factors	Df	Sum of squares	Adjusted MS	P-value	Relative contribution
Temperature of tempering	3	35.667	11.8889	<0.001	48.41
Oil viscosity	2	36.167	18.0833	<0.001	49.09
Error	6	1.833	0.3056
Total	11	73.667

The temperature of tempering is the biggest contributing parameter on the fluctuation of hardness, with a relative contribution of 91%, and can be seen from the ANOVA findings shown in Table 5. For the range of values taken into consideration for the current investigation, it is determined that the relationship between oil viscosity and hardness is statistically negligible.

Table 5. ANOVA results for the hardness of the material

Factors	Df	Sum of squares	Adjusted MS	P-value	Relative contribution
Temperature of tempering	3	851	283.667	<0.001	91
Oil viscosity	2	11.17	5.583	0.624	1.2
Error	6	65.5	10.917
Total	11	927.67

Based on the values for impact toughness and hardness that are currently accessible, regression equations are created. The toughness and hardness effects of the components in the range of values taken into consideration for this investigation are well predicted by the regression equations. Equations 1(1) $\mathit{Impact}\mathit{toughness}\left(\mathit{J}\right)=62.81-0.03067\left(\mathit{Temperature}\mathit{of}\mathit{tempering}\right)-0.3045\left(\mathit{Oilvis}cos\mathit{ity}\right)$(1) and 2(2) $\mathit{Hardness}\left(\mathit{Rc}\right)=-25.9+0.1440\left(\mathit{Temperatureoftempering}\right)-0.214\left(\mathit{Oilvis}cos\mathit{ity}\right)$(2) provide regression equations for impact toughness and hardness, respectively.

(1) $\mathit{Impact}\mathit{toughness}\left(\mathit{J}\right)=62.81-0.03067\left(\mathit{Temperature}\mathit{of}\mathit{tempering}\right)-0.3045\left(\mathit{Oilvis}cos\mathit{ity}\right)$(1)

R- Squared = 78%

(2) $\mathit{Hardness}\left(\mathit{Rc}\right)=-25.9+0.1440\left(\mathit{Temperatureoftempering}\right)-0.214\left(\mathit{Oilvis}cos\mathit{ity}\right)$(2)

R- Squared = 85%

where,

Temperature of tempering is in ºC

Oil viscosity is in centistokes

Response surface approach is used for combined optimization to produce the best impact toughness and hardness combination possible. Impact toughness and hardness are both optimized using the “Maximize” method.

RSM optimization values are as follows:

Temperature of tempering = 453ºC

Oil viscosity = 34.7 centistokes

3.4. Confirmation test

The blend oil composition is changed to 60 volume percentage of Pinnay oil and 40 volume percentage of Karanja oil to get the viscosity value of 34.64 centistokes. A viscometer is used to calculate the oil’s viscosity. At 470°C, the tempering procedure is carried out under identical hardening conditions. The experimentally determined viscosity for the oil mixture is 34.64 centistokes. Table 6 lists the hardness and impact toughness measurements made using theoretical (using regression equations 1(1) $\mathit{Impact}\mathit{toughness}\left(\mathit{J}\right)=62.81-0.03067\left(\mathit{Temperature}\mathit{of}\mathit{tempering}\right)-0.3045\left(\mathit{Oilvis}cos\mathit{ity}\right)$(1) and 2(2) $\mathit{Hardness}\left(\mathit{Rc}\right)=-25.9+0.1440\left(\mathit{Temperatureoftempering}\right)-0.214\left(\mathit{Oilvis}cos\mathit{ity}\right)$(2) ) and experimental methods. The findings show that the percentage of error is less than 5%, indicating that the regression equation is the best fit to identify the attributes within the range of vegetable oil viscosity and tempering temperature utilized.

Table 6. Confirmation test results

Properties	Theoretical	Experimental			Average	% Error
		Trial 1	Trial 2	Trial 3
Hardness (Rc)	34.3	33	33	34	33.3	2.9
Impact toughness (J)	37.8	36.5	37	36.5	36.6	1.2

4. Conclusions

The results obtained from the current study have clearly shown a successful method of using various vegetable oil quenchants for heat treatment of 42CrMo4 steel. The microstructure analysis has shown the presence of tempered martensite dominance as the viscosity of the quenchant decreases, that is, Karanja oil quench indicates more martensite than Pinnay oil quench. The higher tempering temperature has resulted in the enhancement of impact toughness without much decrease in hardness of the specimen. The statistical analysis has shown that the relative contribution of tempering temperature and viscosity of the oil quenchant on the impact toughness is almost equal, whereas oil viscosity did not have significant effect on the variation of hardness for the series of values considered for this study. Regression equations found for predicting the hardness and impact toughness of the 42CrMo4 show good fit, which is proved with the confirmation tests. Tempering temperature of 453ºC and oil viscosity of 34.7 centistokes have been determined from the RSM optimization technique for obtaining the optimized values of impact toughness and hardness. The confirmation test conducted showed that the percentage error evaluated from the experimental and theoretical hardness as well as impact toughness properties is within 3%. It showed that the regression model adopted can be best fitted for the steel and quenchant combination for tempering treatment.

Disclosure statement

No potential conflict of interest was reported by the authors.