- Email: [email protected]
Enhancing the corrosion resistance performance of structural steel via a novel deep cryogenic treatment process
Vacuum 159 (2019) 468–475
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Enhancing the corrosion resistance performance of structural steel via a novel deep cryogenic treatment process
T
Srinivasagam Ramesha, B. Bhuvaneshwarib, G.S. Palanic, D. Mohan Lala, K. Mondald, Raju Kumar Guptab,e,∗ a
Department of Mechanical Engineering, College of Engineering Guindy, Anna University, Chennai, 600025, TN, India Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India c Tower Testing Research Station, CSIR-Structural Engineering Research Centre, Chennai, 600043, TN, India d Department of Material Science and Engineering, Indian Institute of Technology, Kanpur, 208016, Uttar Pradesh, India e Center for Nanosciences and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Deep cryogenic treatment Corrosion Structural steel Electrochemical
The present study is conducted to enhance the corrosion resistance of the structural steel samples by subjecting them to deep cryogenic treatment process. After the cryogenic treatment, one set of samples is subjected for low temperature tempering process and the remaining samples were used without any modifications. To understand the metallurgical changes of the treated samples, scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies were performed. Hardness and tensile strength were also measured. Electrochemical study such as potentiodynamic polarization experiment was carried out to analyze the corrosion resistance of the treated steel samples and comparison was made with the untreated sample. The surface characteristics of all the corroded samples were also characterized using atomic force microscopy (AFM). From the results obtained it was inferred that samples subjected to deep cryogenic corrosion treatment were having enhanced corrosion resistance. Hence, it was concluded that deep cryogenic process can be an industrially viable process for enhancing the structural steel for industrial and infrastructural application.
1. Introduction Structural steel is in high demand in many engineering fields such as building construction, power plants, transportation etc. [1]. Many of these systems operate in environments that are prone for corrosion. There has been an evergrowing demand for better servicelife of system components and frames. Because of the rapid rise in the industrial emissions, the environment is becoming more corrosive. Hence, to increase the servicelife of the structural steel components, improving the corrosion resistance is of paramount importance. Deep cryogenic treatment is an add-on process to the conventional heat treatment processes to enhance the durability of the materials mainly the corrosion resistance [2]. In deep cryogenic treatment, the samples are gradually cooled from room temperature to −196 °C and soaked at that temperature for several hours, before ther are gradually brought back to room temperature [3]. Coming to the tempering process, it's always been a topic of debate in the field of deep cryogenic treatment. There are various reasons and suggestions from researcher's point of view on tempering process that whether tempering need to be
∗
conducted before [4] or after deep cryogenic treatment [5,6] of the materials. Deep cryogenic treatment was originally used to increase the hardness and wear resistance of cutting tools in order to improve their service life [7]. Later, it has been reported that deep cryogenic treatment can be used to improve tensile and bending strengths [8–11], fatigue resistance [12–16], abrasion resistance [17,18] as well as corrosion resistance [19]. Barron et al. [19] have studied the effect of deep cryogenic treatment on the corrosion resistance of 316 SS, 410 SS, 4142 Cr-Mo, S-2 tool steel and M − 1 material and concluded that corrosion resistance improves at varying degrees for different materials. Zhu et al. [20] have investigated the same on medium melting point castable alloy and reported improvement in corrosion resistance. Amin akhbarizadeh et al. [2] have studied the effect of magnetic field during deep cryogenic treatment on a 1.2080 tool steels and reported that there is an increase in corrosion resistance with increase in carbide precipitation. Wang et al. [21] have investigated the influence of deep cryogenic treatment of stainless steel towards the intergranular corrosion in an AISI 304 and found some improvement in intergranular corrosion resistance.
Corresponding author. Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India. E-mail addresses: [email protected] (B. Bhuvaneshwari), [email protected] (D. Mohan Lal), [email protected] (R.K. Gupta).
https://doi.org/10.1016/j.vacuum.2018.10.080 Received 17 August 2018; Received in revised form 21 October 2018; Accepted 31 October 2018 Available online 02 November 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 159 (2019) 468–475
S. Ramesh et al.
on corrosion resistance of metals. From the detailed review of the published literature, it is observed that deep cryogenic treatment can be used to reduce corrosion resistance and no previous research work is reported on structural steel subjected to DCT along with the detailed microstructural study towards understanding their corrosion behavior. The present study of deep cryogenic treatment of structural steel towards enhancing corrosion resistance is novel and will be of significant importance to the industry. Generally, to prevent corrosion in any material, either the material is coated with a passive layer which will protect the surface of the material only or use alloys with corrosion resitant additives which will addup the material cost significantly. But deep cryogenic treatment will enhance the corrosion resistance of the same material throughout crosssection which is quite economical compared to coatings or alloying. The objectives of the study are i) to study the effect of deep cryogenic treatment on corrosion resistance without affecting tensile strength and hardness of the structural steel ii) to understand the underlying mechanisms for the improvement in corrosion resistance iii) to determine whether tempering is needed after deep cryogenic treatment or not.
Cryogenic treatment has been used by various authors to reduce the corrosion effect of the materials thereby extend service life of the materials [21]. Ali Aamir et al. [22] have compared the effect of annealing temperature, cryogenic temperature and deep cryogenic treatment on an aluminium alloy AA5083. It has been found that increasing annealing temperature results in decrease in strength and increase in intergranular corrosion resistance. At cryogenic temperature of −173 °C failure modes have been observed to be a mixture of transgranular and intergranular fractures. Deep cryogenic treatment leads to a slight reduction in strength and hardness, with an increase in ductility and intergranular corrosion resistance.Wei Wang et al. [23] have studied corrosion behavior of an AISI 420 and an AISI 52100 steels in 3.5% NaCL solution and 0.25% NaCl + NaHCO3 solution using electrochemical analysis methods. No significant change in corrosion behavior after deep cryogenic treatment in both the samples exposed to 3.5% NaCl solution could be noticed. However, upon the exposure to 0.25% NaCl + NaHCO3, both the samples have shown resistance to pitting corrosion. The reduction in the retained austenite due to deep cryogenic treatment results in resistance to pitting corrosion. Xiaoyuan Gong et al. [24] have studied the effect of deep cryogenic treatment on the weld specimen of an AZ61 magnesium alloy. Weld specimens were subjected to deep cryogenic treatment for different soaking periods at a constant soaking temperature of −180 °C. Microhardness and corrosion resistance increased with increase in soaking period. It was also suggested that the refined organization of α and β phase led to increased β phase and uniform precipitation, which resulted in the enhancement of corrosion resistance and micro-hardness. G. Hemath Kumar et al. [25] have studied the deep cryogenic treatment effect on the composite material for automotive air conditioning system. The composite was made from mixing epoxy resin and aluminium and then reinforcing glass fibre using hand-lay method. Deep cryogenic treatment was performed at a soaking temperature of −180 °C cooling rate of 1 °C/min and soaking period of 24 h. It was found that the increase in hardness by 13.64% whereas corrosion resistance improved by a factor of 2.65. Yang Gao et al. [26] have experimented WC—Fe—Ni cemented carbides under deep cryogenic treatment performed at −196 °C for soaking periods of 2, 12 and 24 h. The study was performed to understand the effect of deep cryogenic treatment on the microstructure and mechanical properties like wear rate, friction coefficient and corrosion resistance for WC—Fe—Ni cemented carbides. It was inferred from the studies that with prolonged cryogenic treatment, there was a phase transformation in the binder phase from γ-(Fe,Ni) to α-(Fe,Ni) phase. Hence, the α-(Fe,Ni) phase showed an increase from 12.7% to 86.8% (wt.%) due to deep cryogenic treatment. The study showed an increase in hardness of 20%, transverse rapture strength by 7.7%, decrease in wear rate and friction coefficient by almost 56% and by 17.2%, respectively. Further study made on WC—Fe—Ni cemented carbides accounted a slight decrease in corrosion resistance. Ilyas Uygura et al. [27] have studied the corrosion behavior of an AISI D3 steel after deep cryogenic treatment. Comparison has been made for the samples subjected to different types of treatment methods, such as heat treatment, heat treatment with different intervals of deep cryogenic treatment, with and without tempering. It was concluded that the samples subjected for heat treatment followed by tempering and as well as deep cryogenic treatment reduces the corrosion resistance of the samples. Y. Cai et al. [28] have studied the effect of deep cryogenic treatment on welded specimen of AISI304 austenitic stainless steel and the samples have been welded using activating flux tungsten inert gas weld. There has been an increase in the carbide phase and decrease in the grain size of the welded joints. The grain refinement in the weld has resulted in increased strength and micro-hardness as well as precipitation of chromium carbides adjacent to austenite grain boundaries leading to increase the susceptibility to intergranular corrosion. Thus, there are various positive and negative effects of cryogenic treatment
2. Experimental details 2.1. Sample preparation and cryogenic treatment The structural steel used for the present study was first cut into sample of size of 15 × 15 × 10 mm and machined as per ASTM standard E 8–04 for tensile test [24]. The chemical composition determined from optical emission spectroscopy was as follows C - 0.24%, Mn 0.62%, Si - 0.16%, S - 0.049%, P – 0.049%, Ni – 0.09%, Cr – 0.059, Mo – 0.030, Fe- 98.703%. The samples were designated into three sets, such as “untreated” sample (without any treatment), “DCT” samples which are going to be subjected for deep cryogenic treatment and the third one is “DCT + LT” samples, which are going to be subjected to deep cryogenic treatment followed by low temperature tempering. Deep cryogenic treatment was carried out in a cryogenic processor (A.C.I. CP200vi, Applied Cryogenics Inc., Burlington, MA, USA). The cryo-processor consisted of a thermally insulated (super insulation) chamber, which was connected to the liquid nitrogen (LN2) cylinder through a vacuum insulated hose. In this set up, the liquid nitrogen was carefully administered into the chamber through a solenoid valve operated using a micro controller. The temperature inside the chamber was sensed by thermocouple and based on the output the temperature controller operated the solenoid valve, which regulated the liquid nitrogen flow. The spiral heat exchanger and blower built-in was used to regulate and circulate the liquid nitrogen inlet and outlet and chamber temperature was accurately maintained during operation of the chamber. In the present study, the samples were first slowly cooled from room temperature to −186 °C for 3.5 h, followed by soaking for 24 h at the cryogenic temperature. Then, the samples were gradually brought back to room temperature in 6 h. After deep cryogenic treatment, some samples were subjected for tempering at a temperature of 200 °C for a period of 2 h. Finally, the samples undergone the above processes were kept inside the desiccators to keep them dust free. 2.2. Electrochemical corrosion test After the cryogenic and tempering processes as mentioned before, the samples were subjected for electrochemical test to understand the corrosion behavior. As per ASTM standard G3-89 [29] the electrochemical corrosion test was conducted. The samples were polished with 220, 320, 400, 600, 800, 1000, 1200 grit finish using a rotating wheel mounted with silicon carbide sheet. After that the samples were polished on a linen cloth using 0.5 μm-1.0 μm diamond paste then finished by polishing on velvet cloth and kerosene was used as coolant. The 469
Vacuum 159 (2019) 468–475
S. Ramesh et al.
polished samples were swabbed using deionized water and then with acetone. After that, the samples were dipped in a beaker containing acetone and ultra sonicated for 5 min. Finally, the samples were swabbed with acetone and deionized water. The electrochemical corrosion study was conducted in a three electrode cell assembly using electrochemical workstation (Autolab PGSTAT302 N). The specimen was mounted on a Teflon specimen holder with an exposure area of 1 cm2. The as-polished steel samples were used as working, a saturated calomel electrode and platinum electrode were as reference and counter electrodes, respectively. Electrolyte used was 3.5% of NaCl aqueous solution and Luggin probe was used to control the IR drop. Before starting the experiment the working electrodes are immersed in the electrolyte solution for 30 min to stabilize the open circuit potential (OCP) of the system. Potentiodynamic polarization study was conducted with the applied potential from ± 250 mV with respect to the OCP with a constant scan rate of 1 mV/s. Tafel extrapolation method was used to determine corrosion current ‘ICorr’ and corrosion potential ‘Ecorr’ and the corresponding corrosion rate was calculated.
corrosion effect [33]. Hence, durability of the steel can be ensured by subjecting them to deep cryogenic treatment and plausible corrosion protection.
2.3. Characterization methods
3.3. Hardness test
The untreated sample and the samples subjected for cryogenic and tempering processes were analyzed by XRD measurements using a Bruker D2 phaser diffractometer using Cu-Kα radiation (30 Kv, 10 mA) with step size of 0.02 with 2θ varying from 5 to 80° as per ASTM standard E 975–03 [30]. Vickers hardness test of the samples was performed as per ASTM standard E 92–82 [31]. The test was carried out using Vickers hardness tester-wolpert group with diamond indenter. The load of 1 kgf with a dwell period of 15 sec was applied during the conduct of experiment. The tensile strengths of all the samples were determined as per ASTM standard E 8–04 [32] using an universal testing machine (Fuel Instruments & Engineers Pvt. Ltd) at a straining/ piston speeds (at no load) of 0–80 mm/min. During the test, the tensile load was gradually increased till the fracture occurs in the material. Three tests were conducted for each set of condition i.e untreated, DCT & DCT + LT for seeing reproducibility. In order to understand the morphological changes of the samples after the treatment processes carried out, scanning electron microscopic (SEM) studies were conducted using (hitachi 5x to 300,000x). The samples were initially mirror polished and 2% nital solution was used for etching the samples. SEM studies also conducted for all the samples after corrosion. Further, microstructural features of the corroded samples after the electrochemical tests were carried out. The atomic force microscopic (AFM) studies were performed in non-contact mode with the help of XE-70 PARK SYSTEM. The samples to be analyzed were finely cleaned after the electrochemical experiment and then dried in hot air. Comparisons were also made with untreated samples to understand the corrosion resistance effect due to the treatment methods carried out.
Fig. 3 shows the Vickers hardness values of the untreated, DCT + LT and DCT samples. The hardness for the DCT and DCT + LT samples has increased as compared to that of the untreated sample. Among the DCT and DCT + LT samples, an increase in hardness is found maximum for the DCT sample. The decrease in hardness of DCT + LT samples may be due to the effect of low temperature tempering. During deep cryogenic treatment the structures of the systems contacts and also creates few defects including twins and dislocations. These defects act as an attracting centre for carbide nucleations [2] and helps in the improvement in hardness of the material. In the case of DCT + LT samples, due to the low temperature tempering, the carbide nucleations might have slightly disturbed which resulted in the reduced hardness strength compared to DCT samples. It was also believed that the retained austenite would have been more in terms of percentage in DCT samples than in the DCT + LT samples, which have provided additional support for the enhanced hardness of the samples. SEM studies also supports for the increases in hardness for the DCT materials as it attained the uniform secondary carbide precipitation in its structure.
3.2. Phase analysis The XRD patterns of all the samples are shown in Fig. 2. There has been an increase in intensity of martensite peak and carbide in DCT and DCT + LT samples compared to untreated sample. Also absence of austenite in the DCT and DCT + LT samples were noticed. The peak broadening has occurred in DCT and DCT + LT samples exhibiting the information on the possibility of the strain induced inside the sample due to deep cryogenic treatment [34]. It may be argued that the enhancement of martensite intensity is the result of defects such as twins and dislocations created inside the samples during the deep cryogenic processes of the materials which also helped in refinement of carbide phase refinement during the deep cryogenic process. SEM study supports the increase of uniform carbide deposition in DCT samples than in DCT + LT samples.
3.4. Tensile strength evaluation
3. Results and discussion
The ultimate tensile strength and percentage of elongation of all the samples were shown in Figs. 4 and 5. The results suggest that both ultimate tensile strength and the percentage elongation have increased to some extent in DCT and DCT + LT samples. Interestingly, percentage of elongation for the DCT + LT and DCT samples have also increased (∼7%). It may be due to the enhanced uniform precipitation of secondary carbide in the cryogenically treated samples, which would have facilitated for improvement in tensile strength of the samples. Also tempering did not affected the tensile strength of DCT + LT samples.
3.1. Morphological analysis
3.5. Corrosion test
Fig. 1(a, b and c) show the SEM mcirographs of three sets of samples (untreated, DCT + LT and DCT), respectively. The microstructure of the untreated sample consists of ferrite phase as indicated by arrowhead in Fig. 1(a). It is clear from the SEM study that there is an increase in pearlite phase in the microstructure of DCT + LT (Fig. 1(b)) and DCT (Fig. 1(c)) samples. It is evidenced from the SEM images that uniform and homogeneous secondary carbide precipitation in the microstructures of the DCT samples as indicated by arrow head were more than in the DCT + LT samples. It is important to know the fact that pearlite once uniformly dissolved in the matrix, would tend to form an uniform film causing stable matrix and easier passivation from
Fig. 6 shows the dynamic polarization plots for all three sets of samples. The corrosion current (Icorr), corrosion potential (Ecorr) and the corrosion rate of samples are shown in Table 1. It can be noted that the corrosion currents for the DCT and DCT + LT samples have decreased considerably as compared to the untreated sample. Subsequently, the corrosion rates of the DCT and DCT + LT samples have reduced considerably. The corrosion rate of the DCT sample is minimum, whereas, the corrosion rate of the untreated sample is higher than that of the DCT + LT sample. It is believed that the increase in corrosion resistance was due to the uniform and homogenous carbide precipitation found in the microstructure of DCT samples because of the 470
Vacuum 159 (2019) 468–475
S. Ramesh et al.
Fig. 1. SEM images of a) untreated sample b) DCT + LT sample c) DCT sample.
Fig. 2. XRD of Untreated, DCT and DCT + LT sample.
leads to an improvement in corrosion resistance [2]. Also as evident from the SEM results, pearlite formation also helped the DCT samples towards enhancing the corrosion resistance by passive layer formation. It is believed that at extremely low temperature of deep cryogenic treatment, the atomic level movements of system slow down; it results in lower energy of the system which makes them chemically less reactive to the environment. During deep cryogenic treatment, the carbon atoms jump to nearby defects, which also endure a high degree of contraction in the materials and further improves the carbide percentages. The lower corrosion resistance of DCT + LT samples compared to DCT samples was probably due to the increase in atomic level movement of the system because of the post tempering process. The results also infer that when cementite gets oxidized naturally at room
deep cryogenic treatment. The grain boundaries are chemically more active and being highenergy locations, they are more susceptible to corrosion when compared to grain faces [35]. Further DCT also promotes the precipitation of secondary fine carbides homogenously throughout the material [36], resulting in an increased corrosion resistance. It has been reported [19] that the DCT samples experienced fine and homogenous precipitation of secondary carbide. This further reduces the area for the diffusion of NaCl into the metal [19,37]. Thus exposures of the adjacent grains of the microstructure are prevented from corrosion attack. In agreement with the literature, in the present study also, there is a finer precipitation of carbides in DCT & DCT + LT samples compared to that of untreated samples. So it is evident that precipitation of fine carbides 471
Vacuum 159 (2019) 468–475
S. Ramesh et al.
However, in the case of DCT samples, the corrosion resistance was high due to the uniform precipitation of carbides as the nucleation centre for carbide precipitation was kept intact. To ensure the durability of the materials, it is important to sustain the systems energy lower. Towards this, it is suggested that during the tempering process of materials, the temperature should be controlled or this process can be completely eliminated, so that the systems energy level will not be disturbed. Hence, the present study reveals that the deep cryogenic treatment of the system alone brought down the individual constituent of the system into their most stable state [36], resulted in enhanced corrosion resistance of DCT samples. SEM analysis of the corroded samples were shown in Fig. 7. It was evident from the microstructure of the samples that more pits are generated (as indicated with arrow mark) during corrosion attack in the untreated and DCT + LT samples. In the case of DCT samples a uniform secondary carbide precipitation helped in forming a stable passive layer on the steel during corrosion, which have helped them from corrosion attack. Hence corrosion resistance was comparatively increased in DCT samples than in untreated and DCT samples. The another possibility for the less corrosion resistance occurred by DCT + LT samples compared to DCT samples would be due to the lack of retained austenite, which made the DCT + LT samples more vulnerable to corrosion attack.
Fig. 3. Hardness data for all the samples.
3.6. Surface properties analysis Figs. 8 and 9 represent two and three dimensional AFM images of corroded untreated, DCT and DCT + LT samples, respectively. It can be clearly noticed from the AFM images that the corrosion products formed on the surface are uniform in the DCT samples, whereas nonuniformity is noticed for untreated samples. Little less non-uniformity is present in the DCT + LT sample too as compared to the untreated sample. From the peak profile analysis carried out for all the samples as shown in Fig. 10, it is observed that the corresponding peak profile values for the corroded samples untreated, DCT and DCT + LT are in the range of −100 nm to 100 nm, −20 nm–20 nm and −40 nm to 40 nm, respectively. The surface profile parameters such as average roughness (Ra), maximum peak to valley distance (Rpv), and root mean square roughness (Rq) of the samples were tabulated in Table 2. The roughness of the corroded DCT sample is very less as compared to the corroded untreated sample. Also, the roughness of corroded DCT + LT samples is slightly greater than DCT samples but considerably less than untreated samples. It is an indication that, due to tempering effect, the corrosion attack on the surface of DCT + LT samples has increased. The roughness value of the DCT samples indicates least vulnerability to corrosion attack. It may be due to the fact that in the case of untreated samples, the less corrosion resistance is due to the absence of fine secondary carbide precipitation. However, in other cases, the corrosion protection was ensured by the treatment processes carried out. This was a clear indication from the roughness parameters obtained for the untreated, DCT and DCT + LT samples, which also indicate the depth of attack of the samples. Other studies such as polarization, SEM and XRD have also been in agreement with the obtained AFM result of DCT samples. SEM study supports for the finding because the carbide nucleation would have disturbed by the low temperature tempering process which resulted in slightly non-homogeneous carbide precipitation in DCT + LT samples. Hence, it can be argued that the passive layer which usually forms on the metal surface would have strengthened in the case of DCT samples compared to other samples that might have contributed in the protection of metal surface from corrosion. So, the deep cryogenic processing of metal was having positive effect on controlling the samples from corrosion attack without losing its mechanical properties. Hence, it is concluded that the deep cryogenic treatment can be suggested for steel industry to enhance the corrosion resistance of the structural steel for using them in special applications such as aerospace, freeze-thaw region, tall structures construction etc. It also promotes the
Fig. 4. Tensile strength data for all the samples.
Fig. 5. Elongation of all the samples.
temperature, the oxides formed are mainly γ-FeOOH and Fe3O4, which are amorphous in nature and have high corrosion resistance. Whereas, when it gets oxidized at high temperature, it ended up in forming Fe3O4 and α-Fe2O3 oxides, lead to lower corrosion resistance of system [33]. Thus, in spite of carbide precipitation in DCT + LT samples, a lower corrosion resistance was observed compared to DCT samples. 472
Vacuum 159 (2019) 468–475
S. Ramesh et al.
Fig. 6. Tafel curves of Untreated, DCT and DCT + LT samples.
system and subject them for the same studies, the results may be different. In that case detailed study is needed to understand the systems performance.
Table 1 Potentiodynamic polarization data of the all the samples. S.No.
Sample Details
Icorr (μA)
Ecorr (mV)
Corrosion rate (mm/y)
1 2 3
untreated DCT DCT + LT
121.330 6.129 61.159
−615.270 −650.690 −624.180
1.4092 0.0712 0.7106
4. Conclusion The present study carried out on structural steel suggests that deep cryogenic treatment can be used to improve the corrosion resistance property.
steel structure construction in infrastructural application. However, the present study is only applicable for the selected grade of steel which is used in the present study. So, when we select the different grade of steel
• Deep cryogenic treatment had a positive effect on the corrosion
resistance, whereas, tempering done after deep cryogenic treatment
Fig. 7. SEM images of samples after corrosion a) untreated b) DCT c) DCT + LT. 473
Vacuum 159 (2019) 468–475
S. Ramesh et al.
Fig. 8. Two Dimensional AFM images of all the samples.
Fig. 9. Three dimensional AFM images of all the samples.
Fig. 10. Peak profile analysis of all the samples.
carbide precipitation.
Table 2 Roughness data of all the samples. Sample Details
Rpv
Rq
Ra
untreated DCT DCT + LT
437.695 71.301 94.283
46.483 8.657 15.313
32.888 6.813 12.123
Hence, from the results of the present study, the deep cryogenic treatment of the structural steel samples can be recommended to enhance or improve the corrosion resistance of structural steel for using them in relevant industrial or infrastructural applications. Acknowledgements
• • • • •
showed negative effects. Hence, tempering is not suggested for this material after deep cryogenic treatment. It was also found that the deep cryogenic treatment process for steel material enhances the corrosion resistance of structural steel by 95%. Increase in corrosion resistance in the material was due to uniform and homogenous carbide precipitation in the microstructure of DCT. Hardness and tensile strength test results inferred that there were no negative effects due to deep cryogenic treatment. SEM and AFM studies support towards uniform layer formation and corrosion resistance enhancement of the DCT samples. In the case of DCT + LT samples, due to low temperature tempering process, the system attained microstructural weakness towards corrosion resistance due to slightly non-homogeneous secondary
Authors acknowledge IIT Kanpur, CSIR –SERC and Anna University, Chennai, India for the constant support and encouragement provided for the present study. References [1] Nagaraj M and B.Ravisankar, Enhancing the strength of structural steel through severe plastic deformation based thermomechanical treatment, Mater. Sci. Eng., Available online 25 September 2018. https://doi.org/10.1016/j.msea.2018.09.095. [2] Amin Akhbarizadeh, Kamran Amini, Sirus Javadpour, Effects of applying an external magnetic field during the deep cryogenic heat treatment on the corrosion resistance and wear behavior of 1.2080 tool steel, mater design 41 (2012) 114–123. [3] P. Baldissera, C. Delprete, Deep cryogenic treatment: a bibliographic review, Open Mech. Eng. J. 2 (2008) 1–11. [4] D. Yun, L. Xiaoping, X. Hongshen, Deep cryogenic treatment of high-speed steel and its mechanism, Heat Treat. Metals 3 (1998) 55–59.
474
Vacuum 159 (2019) 468–475
S. Ramesh et al.
[5] S.H. Avner, Introduction to Physical Metallurgy, Tata McGraw-Hill Publishing Co. Ltd., New Delhi, 2003, pp. 305–307. [6] L.H. Vanvlack, Elements of material science and engineering, addison-wesley series in metall, Mater. Eng. 6 (1998) 316–320. [7] D.N. Collins, J. Dormer, Deep cryogenic treatment of a D2 cold-worked tool steel, Heat Treat. Met. 24 (3) (1997) 71–74. [8] Chuang-xian Xiong, Xin-ming Zhang, Yun-lai Deng, Yang Xiao, Zhen-zhen Deng, Buxiang Chen, Effects of cryogenic treatment on mechanical properties of extruded Mg–Gd–Y–Zr (Mn) alloys, J Central South UnivTechnol 14 (2007) 305–309. [9] P. Baldissera, Fatigue scatter reduction through deep cryogenic treatment on the 18NiCrMo5 carburized steel, Mater. Des. 30 (2009) 3636–3642. [10] A. Bensely, D. Senthilkumar, D. MohanLal, G. Nagarajan, A. Rajadurai, Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17, Mater. Char. 58 (2007) 485–491. [11] M. ArockiaJaswin, D. Mohan Lal, Effect of cryogenic treatment on the tensile behaviour of En 52 and 21-4N valve steels at room and elevated temperatures, Mater. Des. 32 (2010) 2429–2437. [12] P. Johan Singh, S.L. Mannan, T. Jayakumar, D.R.G. Achar, Fatigue life extension of notches in AISI 304L weldments using deep cryogenic treatment, Eng. Fail. Anal. 12 (2005) 263–271. [13] S. Zhirafar, A. Rezaeian, M. Pugh, Effect of cryogenic treatment on the mechanical properties of 4340 steel, J. Mater. Process. Technol. 186 (2007) 298–303. [14] Nirmal S. Kalsi, Rakesh Sehgal, Vishal S. Sharma, Cryogenic Treatment of Tool Materials: A Review Materials and Manufacturing Processes 25 (2010) 1077–1100. [15] P. Johan Singh, B. Guha, Fatigue life improvement of AISI 304L cruciform welded joints by cryogenic treatment, Eng. Fail. Anal. 10 (2003) 1–12. [16] P. Baldissera, Fatigue scatter reduction through deep cryogenic treatment on the 18NiCrMo5 carburized steel, Mater. Des. 30 (2009) 3636–3642. [17] H. Hao-huai Liu, J. Wang, H. Yang, B. Shen, Effects of cryogenic treatment on microstructure and abrasion resistance of CrMnB high-chromium cast iron subjected to sub-critical treatment, Mater. Sci. Eng. 478 (2008) 324–328. [18] H. Hao-huai Liu, J. Wang, B. Shen, H. Yang, S. Gao, S. Huang, Effects of deep cryogenic treatment on property of 3Cr13Mo1V1.5 high chromium cast iron, Mater. Des. 28 (2007) 1059–1064. [19] R.F. Barron, R.H. Thompson, Effect of Cryogenic treatment on corrosion resistance, Advances in Cryogenic Engineering Materials 36 (1990) 1375–1379. [20] Z. Zhu, J. Zhao, X. Huang, Effects of cryotreat on the corrosion resistance of the medium melting point castable alloy, West China J. Stomatol. 20 (5) (2002) 316–319. [21] Wang Qiong-Qi, Wang Wei-Ze, Xuan Fu-Zhen, Tu Shan-Tung, A new method improving intergranular corrosion resistance of AISI 304 stainless steel, FM2008 – Evaluation, Inspection and Monitoring of Structural Integrity (2008) 411–416. [22] Aamir Ali, Lei Pan, Lixiang Duan, Zengmin Zheng, Effects of deep cryogenic
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31] [32] [33] [34] [35] [36] [37]
475
treatment, cryogenic and annealing temperatures on mechanical properties and corrosion resistance of AA5083 aluminium alloy, Int. J. Microstruct. Mater. Prop. 11 (2016) 5. Wei Wang, Venkateswaran Srinivasan, Sri Siva, Bensely Albert, Mohan Lal, Akram Alfantazi, Corrosion behavior of deep cryogenically treated AISI 420 and AISI 52100 steel, Corrosion 70 (7) (2014) 708–720. Xiaoyuan Gong, Zhisheng Wu, Fei Zhao, Effect of deep cryogenic treatment on the microstructure and the corrosion resistance of AZ61 magnesium alloy welded joint, Metals 7 (2017) 179. G. Hemath Kumar, H. Mohit, Rajesh Purohit, Effect of deep cryogenic treatment on composite material for automotive Ac system, 5th international conference of materials processing and characterization (ICMPC 2016), Mater. Today: Proceedings 4 (2017) 3501–3505. Yang Gao, Bing-Hui Luo, Zhen-hai Bai, Bin Zhu, Sheng Ouyang, Effects of deep cryogenic treatment on the microstructure and properties of WC—Fe—Ni cemented carbides, Int. J. Refract. Metals Hard Mater. 58 (2016) 42–50. Ilyas Uygura, Husnu Gerengib, Yusuf Arslanc, Mine Kurtayb, The effects of cryogenic treatment on the corrosion of AISI D3 steel, Mater. Res. 18 (3) (2015) 569–574. Y. Cai, Z. Luo &Y. Zeng, Influence of deep cryogenic treatment on the microstructure and properties of AISI304 austenitic stainless steel A-TIG weld, Sci. Technol. Weld. Join. 22 (3) (2017) 236–243. ASTM Standard G3-89, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, Annual book of standards, 2004. ASTM Standard E975-03, Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation, Annual book of standards, 2004. ASTM Standard E92-82, Standard Test Method for Vickers Hardness of Metallic Materials, Annual book of standards, 2004. ASTM Standard E 8-04, Standard Test Methods for Tension Testing of Metallic Materials, Annual book of standards, 2004. M. Ferhat, A. Benchettara, S.E. Amara, D. Najjar, Corrosion behaviour of Fe-C alloys in a sulfuric medium, J. Mater. Environ. Sci. 5 (4) (2014) 1059–1068. J. Mazur, Investigation on austenite and martensite subjected to very low temperatures, Cryogenics 4 (1964) 36. Mars G. Fontana, Corrosion Engineering, Tata McGraw-Hill, New Delhi, India, 2005. SusheelKalia, Cryogenic processing: a study of materials at low temperatures, J. Low Temp. Phys. 158 (2010) 934–945. H. Yumoto, Y. Nagamine, J. Nagahama, M. Shimotomai, Corrosion and stability of cementite films prepared by electron shower, Vacuum 65 (2002) 527–531.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Enhancing the corrosion resistance performance of structural steel via a novel deep cryogenic treatment process
T
Srinivasagam Ramesha, B. Bhuvaneshwarib, G.S. Palanic, D. Mohan Lala, K. Mondald, Raju Kumar Guptab,e,∗ a
Department of Mechanical Engineering, College of Engineering Guindy, Anna University, Chennai, 600025, TN, India Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India c Tower Testing Research Station, CSIR-Structural Engineering Research Centre, Chennai, 600043, TN, India d Department of Material Science and Engineering, Indian Institute of Technology, Kanpur, 208016, Uttar Pradesh, India e Center for Nanosciences and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Deep cryogenic treatment Corrosion Structural steel Electrochemical
The present study is conducted to enhance the corrosion resistance of the structural steel samples by subjecting them to deep cryogenic treatment process. After the cryogenic treatment, one set of samples is subjected for low temperature tempering process and the remaining samples were used without any modifications. To understand the metallurgical changes of the treated samples, scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies were performed. Hardness and tensile strength were also measured. Electrochemical study such as potentiodynamic polarization experiment was carried out to analyze the corrosion resistance of the treated steel samples and comparison was made with the untreated sample. The surface characteristics of all the corroded samples were also characterized using atomic force microscopy (AFM). From the results obtained it was inferred that samples subjected to deep cryogenic corrosion treatment were having enhanced corrosion resistance. Hence, it was concluded that deep cryogenic process can be an industrially viable process for enhancing the structural steel for industrial and infrastructural application.
1. Introduction Structural steel is in high demand in many engineering fields such as building construction, power plants, transportation etc. [1]. Many of these systems operate in environments that are prone for corrosion. There has been an evergrowing demand for better servicelife of system components and frames. Because of the rapid rise in the industrial emissions, the environment is becoming more corrosive. Hence, to increase the servicelife of the structural steel components, improving the corrosion resistance is of paramount importance. Deep cryogenic treatment is an add-on process to the conventional heat treatment processes to enhance the durability of the materials mainly the corrosion resistance [2]. In deep cryogenic treatment, the samples are gradually cooled from room temperature to −196 °C and soaked at that temperature for several hours, before ther are gradually brought back to room temperature [3]. Coming to the tempering process, it's always been a topic of debate in the field of deep cryogenic treatment. There are various reasons and suggestions from researcher's point of view on tempering process that whether tempering need to be
∗
conducted before [4] or after deep cryogenic treatment [5,6] of the materials. Deep cryogenic treatment was originally used to increase the hardness and wear resistance of cutting tools in order to improve their service life [7]. Later, it has been reported that deep cryogenic treatment can be used to improve tensile and bending strengths [8–11], fatigue resistance [12–16], abrasion resistance [17,18] as well as corrosion resistance [19]. Barron et al. [19] have studied the effect of deep cryogenic treatment on the corrosion resistance of 316 SS, 410 SS, 4142 Cr-Mo, S-2 tool steel and M − 1 material and concluded that corrosion resistance improves at varying degrees for different materials. Zhu et al. [20] have investigated the same on medium melting point castable alloy and reported improvement in corrosion resistance. Amin akhbarizadeh et al. [2] have studied the effect of magnetic field during deep cryogenic treatment on a 1.2080 tool steels and reported that there is an increase in corrosion resistance with increase in carbide precipitation. Wang et al. [21] have investigated the influence of deep cryogenic treatment of stainless steel towards the intergranular corrosion in an AISI 304 and found some improvement in intergranular corrosion resistance.
Corresponding author. Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India. E-mail addresses: [email protected] (B. Bhuvaneshwari), [email protected] (D. Mohan Lal), [email protected] (R.K. Gupta).
https://doi.org/10.1016/j.vacuum.2018.10.080 Received 17 August 2018; Received in revised form 21 October 2018; Accepted 31 October 2018 Available online 02 November 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Vacuum 159 (2019) 468–475
S. Ramesh et al.
on corrosion resistance of metals. From the detailed review of the published literature, it is observed that deep cryogenic treatment can be used to reduce corrosion resistance and no previous research work is reported on structural steel subjected to DCT along with the detailed microstructural study towards understanding their corrosion behavior. The present study of deep cryogenic treatment of structural steel towards enhancing corrosion resistance is novel and will be of significant importance to the industry. Generally, to prevent corrosion in any material, either the material is coated with a passive layer which will protect the surface of the material only or use alloys with corrosion resitant additives which will addup the material cost significantly. But deep cryogenic treatment will enhance the corrosion resistance of the same material throughout crosssection which is quite economical compared to coatings or alloying. The objectives of the study are i) to study the effect of deep cryogenic treatment on corrosion resistance without affecting tensile strength and hardness of the structural steel ii) to understand the underlying mechanisms for the improvement in corrosion resistance iii) to determine whether tempering is needed after deep cryogenic treatment or not.
Cryogenic treatment has been used by various authors to reduce the corrosion effect of the materials thereby extend service life of the materials [21]. Ali Aamir et al. [22] have compared the effect of annealing temperature, cryogenic temperature and deep cryogenic treatment on an aluminium alloy AA5083. It has been found that increasing annealing temperature results in decrease in strength and increase in intergranular corrosion resistance. At cryogenic temperature of −173 °C failure modes have been observed to be a mixture of transgranular and intergranular fractures. Deep cryogenic treatment leads to a slight reduction in strength and hardness, with an increase in ductility and intergranular corrosion resistance.Wei Wang et al. [23] have studied corrosion behavior of an AISI 420 and an AISI 52100 steels in 3.5% NaCL solution and 0.25% NaCl + NaHCO3 solution using electrochemical analysis methods. No significant change in corrosion behavior after deep cryogenic treatment in both the samples exposed to 3.5% NaCl solution could be noticed. However, upon the exposure to 0.25% NaCl + NaHCO3, both the samples have shown resistance to pitting corrosion. The reduction in the retained austenite due to deep cryogenic treatment results in resistance to pitting corrosion. Xiaoyuan Gong et al. [24] have studied the effect of deep cryogenic treatment on the weld specimen of an AZ61 magnesium alloy. Weld specimens were subjected to deep cryogenic treatment for different soaking periods at a constant soaking temperature of −180 °C. Microhardness and corrosion resistance increased with increase in soaking period. It was also suggested that the refined organization of α and β phase led to increased β phase and uniform precipitation, which resulted in the enhancement of corrosion resistance and micro-hardness. G. Hemath Kumar et al. [25] have studied the deep cryogenic treatment effect on the composite material for automotive air conditioning system. The composite was made from mixing epoxy resin and aluminium and then reinforcing glass fibre using hand-lay method. Deep cryogenic treatment was performed at a soaking temperature of −180 °C cooling rate of 1 °C/min and soaking period of 24 h. It was found that the increase in hardness by 13.64% whereas corrosion resistance improved by a factor of 2.65. Yang Gao et al. [26] have experimented WC—Fe—Ni cemented carbides under deep cryogenic treatment performed at −196 °C for soaking periods of 2, 12 and 24 h. The study was performed to understand the effect of deep cryogenic treatment on the microstructure and mechanical properties like wear rate, friction coefficient and corrosion resistance for WC—Fe—Ni cemented carbides. It was inferred from the studies that with prolonged cryogenic treatment, there was a phase transformation in the binder phase from γ-(Fe,Ni) to α-(Fe,Ni) phase. Hence, the α-(Fe,Ni) phase showed an increase from 12.7% to 86.8% (wt.%) due to deep cryogenic treatment. The study showed an increase in hardness of 20%, transverse rapture strength by 7.7%, decrease in wear rate and friction coefficient by almost 56% and by 17.2%, respectively. Further study made on WC—Fe—Ni cemented carbides accounted a slight decrease in corrosion resistance. Ilyas Uygura et al. [27] have studied the corrosion behavior of an AISI D3 steel after deep cryogenic treatment. Comparison has been made for the samples subjected to different types of treatment methods, such as heat treatment, heat treatment with different intervals of deep cryogenic treatment, with and without tempering. It was concluded that the samples subjected for heat treatment followed by tempering and as well as deep cryogenic treatment reduces the corrosion resistance of the samples. Y. Cai et al. [28] have studied the effect of deep cryogenic treatment on welded specimen of AISI304 austenitic stainless steel and the samples have been welded using activating flux tungsten inert gas weld. There has been an increase in the carbide phase and decrease in the grain size of the welded joints. The grain refinement in the weld has resulted in increased strength and micro-hardness as well as precipitation of chromium carbides adjacent to austenite grain boundaries leading to increase the susceptibility to intergranular corrosion. Thus, there are various positive and negative effects of cryogenic treatment
2. Experimental details 2.1. Sample preparation and cryogenic treatment The structural steel used for the present study was first cut into sample of size of 15 × 15 × 10 mm and machined as per ASTM standard E 8–04 for tensile test [24]. The chemical composition determined from optical emission spectroscopy was as follows C - 0.24%, Mn 0.62%, Si - 0.16%, S - 0.049%, P – 0.049%, Ni – 0.09%, Cr – 0.059, Mo – 0.030, Fe- 98.703%. The samples were designated into three sets, such as “untreated” sample (without any treatment), “DCT” samples which are going to be subjected for deep cryogenic treatment and the third one is “DCT + LT” samples, which are going to be subjected to deep cryogenic treatment followed by low temperature tempering. Deep cryogenic treatment was carried out in a cryogenic processor (A.C.I. CP200vi, Applied Cryogenics Inc., Burlington, MA, USA). The cryo-processor consisted of a thermally insulated (super insulation) chamber, which was connected to the liquid nitrogen (LN2) cylinder through a vacuum insulated hose. In this set up, the liquid nitrogen was carefully administered into the chamber through a solenoid valve operated using a micro controller. The temperature inside the chamber was sensed by thermocouple and based on the output the temperature controller operated the solenoid valve, which regulated the liquid nitrogen flow. The spiral heat exchanger and blower built-in was used to regulate and circulate the liquid nitrogen inlet and outlet and chamber temperature was accurately maintained during operation of the chamber. In the present study, the samples were first slowly cooled from room temperature to −186 °C for 3.5 h, followed by soaking for 24 h at the cryogenic temperature. Then, the samples were gradually brought back to room temperature in 6 h. After deep cryogenic treatment, some samples were subjected for tempering at a temperature of 200 °C for a period of 2 h. Finally, the samples undergone the above processes were kept inside the desiccators to keep them dust free. 2.2. Electrochemical corrosion test After the cryogenic and tempering processes as mentioned before, the samples were subjected for electrochemical test to understand the corrosion behavior. As per ASTM standard G3-89 [29] the electrochemical corrosion test was conducted. The samples were polished with 220, 320, 400, 600, 800, 1000, 1200 grit finish using a rotating wheel mounted with silicon carbide sheet. After that the samples were polished on a linen cloth using 0.5 μm-1.0 μm diamond paste then finished by polishing on velvet cloth and kerosene was used as coolant. The 469
Vacuum 159 (2019) 468–475
S. Ramesh et al.
polished samples were swabbed using deionized water and then with acetone. After that, the samples were dipped in a beaker containing acetone and ultra sonicated for 5 min. Finally, the samples were swabbed with acetone and deionized water. The electrochemical corrosion study was conducted in a three electrode cell assembly using electrochemical workstation (Autolab PGSTAT302 N). The specimen was mounted on a Teflon specimen holder with an exposure area of 1 cm2. The as-polished steel samples were used as working, a saturated calomel electrode and platinum electrode were as reference and counter electrodes, respectively. Electrolyte used was 3.5% of NaCl aqueous solution and Luggin probe was used to control the IR drop. Before starting the experiment the working electrodes are immersed in the electrolyte solution for 30 min to stabilize the open circuit potential (OCP) of the system. Potentiodynamic polarization study was conducted with the applied potential from ± 250 mV with respect to the OCP with a constant scan rate of 1 mV/s. Tafel extrapolation method was used to determine corrosion current ‘ICorr’ and corrosion potential ‘Ecorr’ and the corresponding corrosion rate was calculated.
corrosion effect [33]. Hence, durability of the steel can be ensured by subjecting them to deep cryogenic treatment and plausible corrosion protection.
2.3. Characterization methods
3.3. Hardness test
The untreated sample and the samples subjected for cryogenic and tempering processes were analyzed by XRD measurements using a Bruker D2 phaser diffractometer using Cu-Kα radiation (30 Kv, 10 mA) with step size of 0.02 with 2θ varying from 5 to 80° as per ASTM standard E 975–03 [30]. Vickers hardness test of the samples was performed as per ASTM standard E 92–82 [31]. The test was carried out using Vickers hardness tester-wolpert group with diamond indenter. The load of 1 kgf with a dwell period of 15 sec was applied during the conduct of experiment. The tensile strengths of all the samples were determined as per ASTM standard E 8–04 [32] using an universal testing machine (Fuel Instruments & Engineers Pvt. Ltd) at a straining/ piston speeds (at no load) of 0–80 mm/min. During the test, the tensile load was gradually increased till the fracture occurs in the material. Three tests were conducted for each set of condition i.e untreated, DCT & DCT + LT for seeing reproducibility. In order to understand the morphological changes of the samples after the treatment processes carried out, scanning electron microscopic (SEM) studies were conducted using (hitachi 5x to 300,000x). The samples were initially mirror polished and 2% nital solution was used for etching the samples. SEM studies also conducted for all the samples after corrosion. Further, microstructural features of the corroded samples after the electrochemical tests were carried out. The atomic force microscopic (AFM) studies were performed in non-contact mode with the help of XE-70 PARK SYSTEM. The samples to be analyzed were finely cleaned after the electrochemical experiment and then dried in hot air. Comparisons were also made with untreated samples to understand the corrosion resistance effect due to the treatment methods carried out.
Fig. 3 shows the Vickers hardness values of the untreated, DCT + LT and DCT samples. The hardness for the DCT and DCT + LT samples has increased as compared to that of the untreated sample. Among the DCT and DCT + LT samples, an increase in hardness is found maximum for the DCT sample. The decrease in hardness of DCT + LT samples may be due to the effect of low temperature tempering. During deep cryogenic treatment the structures of the systems contacts and also creates few defects including twins and dislocations. These defects act as an attracting centre for carbide nucleations [2] and helps in the improvement in hardness of the material. In the case of DCT + LT samples, due to the low temperature tempering, the carbide nucleations might have slightly disturbed which resulted in the reduced hardness strength compared to DCT samples. It was also believed that the retained austenite would have been more in terms of percentage in DCT samples than in the DCT + LT samples, which have provided additional support for the enhanced hardness of the samples. SEM studies also supports for the increases in hardness for the DCT materials as it attained the uniform secondary carbide precipitation in its structure.
3.2. Phase analysis The XRD patterns of all the samples are shown in Fig. 2. There has been an increase in intensity of martensite peak and carbide in DCT and DCT + LT samples compared to untreated sample. Also absence of austenite in the DCT and DCT + LT samples were noticed. The peak broadening has occurred in DCT and DCT + LT samples exhibiting the information on the possibility of the strain induced inside the sample due to deep cryogenic treatment [34]. It may be argued that the enhancement of martensite intensity is the result of defects such as twins and dislocations created inside the samples during the deep cryogenic processes of the materials which also helped in refinement of carbide phase refinement during the deep cryogenic process. SEM study supports the increase of uniform carbide deposition in DCT samples than in DCT + LT samples.
3.4. Tensile strength evaluation
3. Results and discussion
The ultimate tensile strength and percentage of elongation of all the samples were shown in Figs. 4 and 5. The results suggest that both ultimate tensile strength and the percentage elongation have increased to some extent in DCT and DCT + LT samples. Interestingly, percentage of elongation for the DCT + LT and DCT samples have also increased (∼7%). It may be due to the enhanced uniform precipitation of secondary carbide in the cryogenically treated samples, which would have facilitated for improvement in tensile strength of the samples. Also tempering did not affected the tensile strength of DCT + LT samples.
3.1. Morphological analysis
3.5. Corrosion test
Fig. 1(a, b and c) show the SEM mcirographs of three sets of samples (untreated, DCT + LT and DCT), respectively. The microstructure of the untreated sample consists of ferrite phase as indicated by arrowhead in Fig. 1(a). It is clear from the SEM study that there is an increase in pearlite phase in the microstructure of DCT + LT (Fig. 1(b)) and DCT (Fig. 1(c)) samples. It is evidenced from the SEM images that uniform and homogeneous secondary carbide precipitation in the microstructures of the DCT samples as indicated by arrow head were more than in the DCT + LT samples. It is important to know the fact that pearlite once uniformly dissolved in the matrix, would tend to form an uniform film causing stable matrix and easier passivation from
Fig. 6 shows the dynamic polarization plots for all three sets of samples. The corrosion current (Icorr), corrosion potential (Ecorr) and the corrosion rate of samples are shown in Table 1. It can be noted that the corrosion currents for the DCT and DCT + LT samples have decreased considerably as compared to the untreated sample. Subsequently, the corrosion rates of the DCT and DCT + LT samples have reduced considerably. The corrosion rate of the DCT sample is minimum, whereas, the corrosion rate of the untreated sample is higher than that of the DCT + LT sample. It is believed that the increase in corrosion resistance was due to the uniform and homogenous carbide precipitation found in the microstructure of DCT samples because of the 470
Vacuum 159 (2019) 468–475
S. Ramesh et al.
Fig. 1. SEM images of a) untreated sample b) DCT + LT sample c) DCT sample.
Fig. 2. XRD of Untreated, DCT and DCT + LT sample.
leads to an improvement in corrosion resistance [2]. Also as evident from the SEM results, pearlite formation also helped the DCT samples towards enhancing the corrosion resistance by passive layer formation. It is believed that at extremely low temperature of deep cryogenic treatment, the atomic level movements of system slow down; it results in lower energy of the system which makes them chemically less reactive to the environment. During deep cryogenic treatment, the carbon atoms jump to nearby defects, which also endure a high degree of contraction in the materials and further improves the carbide percentages. The lower corrosion resistance of DCT + LT samples compared to DCT samples was probably due to the increase in atomic level movement of the system because of the post tempering process. The results also infer that when cementite gets oxidized naturally at room
deep cryogenic treatment. The grain boundaries are chemically more active and being highenergy locations, they are more susceptible to corrosion when compared to grain faces [35]. Further DCT also promotes the precipitation of secondary fine carbides homogenously throughout the material [36], resulting in an increased corrosion resistance. It has been reported [19] that the DCT samples experienced fine and homogenous precipitation of secondary carbide. This further reduces the area for the diffusion of NaCl into the metal [19,37]. Thus exposures of the adjacent grains of the microstructure are prevented from corrosion attack. In agreement with the literature, in the present study also, there is a finer precipitation of carbides in DCT & DCT + LT samples compared to that of untreated samples. So it is evident that precipitation of fine carbides 471
Vacuum 159 (2019) 468–475
S. Ramesh et al.
However, in the case of DCT samples, the corrosion resistance was high due to the uniform precipitation of carbides as the nucleation centre for carbide precipitation was kept intact. To ensure the durability of the materials, it is important to sustain the systems energy lower. Towards this, it is suggested that during the tempering process of materials, the temperature should be controlled or this process can be completely eliminated, so that the systems energy level will not be disturbed. Hence, the present study reveals that the deep cryogenic treatment of the system alone brought down the individual constituent of the system into their most stable state [36], resulted in enhanced corrosion resistance of DCT samples. SEM analysis of the corroded samples were shown in Fig. 7. It was evident from the microstructure of the samples that more pits are generated (as indicated with arrow mark) during corrosion attack in the untreated and DCT + LT samples. In the case of DCT samples a uniform secondary carbide precipitation helped in forming a stable passive layer on the steel during corrosion, which have helped them from corrosion attack. Hence corrosion resistance was comparatively increased in DCT samples than in untreated and DCT samples. The another possibility for the less corrosion resistance occurred by DCT + LT samples compared to DCT samples would be due to the lack of retained austenite, which made the DCT + LT samples more vulnerable to corrosion attack.
Fig. 3. Hardness data for all the samples.
3.6. Surface properties analysis Figs. 8 and 9 represent two and three dimensional AFM images of corroded untreated, DCT and DCT + LT samples, respectively. It can be clearly noticed from the AFM images that the corrosion products formed on the surface are uniform in the DCT samples, whereas nonuniformity is noticed for untreated samples. Little less non-uniformity is present in the DCT + LT sample too as compared to the untreated sample. From the peak profile analysis carried out for all the samples as shown in Fig. 10, it is observed that the corresponding peak profile values for the corroded samples untreated, DCT and DCT + LT are in the range of −100 nm to 100 nm, −20 nm–20 nm and −40 nm to 40 nm, respectively. The surface profile parameters such as average roughness (Ra), maximum peak to valley distance (Rpv), and root mean square roughness (Rq) of the samples were tabulated in Table 2. The roughness of the corroded DCT sample is very less as compared to the corroded untreated sample. Also, the roughness of corroded DCT + LT samples is slightly greater than DCT samples but considerably less than untreated samples. It is an indication that, due to tempering effect, the corrosion attack on the surface of DCT + LT samples has increased. The roughness value of the DCT samples indicates least vulnerability to corrosion attack. It may be due to the fact that in the case of untreated samples, the less corrosion resistance is due to the absence of fine secondary carbide precipitation. However, in other cases, the corrosion protection was ensured by the treatment processes carried out. This was a clear indication from the roughness parameters obtained for the untreated, DCT and DCT + LT samples, which also indicate the depth of attack of the samples. Other studies such as polarization, SEM and XRD have also been in agreement with the obtained AFM result of DCT samples. SEM study supports for the finding because the carbide nucleation would have disturbed by the low temperature tempering process which resulted in slightly non-homogeneous carbide precipitation in DCT + LT samples. Hence, it can be argued that the passive layer which usually forms on the metal surface would have strengthened in the case of DCT samples compared to other samples that might have contributed in the protection of metal surface from corrosion. So, the deep cryogenic processing of metal was having positive effect on controlling the samples from corrosion attack without losing its mechanical properties. Hence, it is concluded that the deep cryogenic treatment can be suggested for steel industry to enhance the corrosion resistance of the structural steel for using them in special applications such as aerospace, freeze-thaw region, tall structures construction etc. It also promotes the
Fig. 4. Tensile strength data for all the samples.
Fig. 5. Elongation of all the samples.
temperature, the oxides formed are mainly γ-FeOOH and Fe3O4, which are amorphous in nature and have high corrosion resistance. Whereas, when it gets oxidized at high temperature, it ended up in forming Fe3O4 and α-Fe2O3 oxides, lead to lower corrosion resistance of system [33]. Thus, in spite of carbide precipitation in DCT + LT samples, a lower corrosion resistance was observed compared to DCT samples. 472
Vacuum 159 (2019) 468–475
S. Ramesh et al.
Fig. 6. Tafel curves of Untreated, DCT and DCT + LT samples.
system and subject them for the same studies, the results may be different. In that case detailed study is needed to understand the systems performance.
Table 1 Potentiodynamic polarization data of the all the samples. S.No.
Sample Details
Icorr (μA)
Ecorr (mV)
Corrosion rate (mm/y)
1 2 3
untreated DCT DCT + LT
121.330 6.129 61.159
−615.270 −650.690 −624.180
1.4092 0.0712 0.7106
4. Conclusion The present study carried out on structural steel suggests that deep cryogenic treatment can be used to improve the corrosion resistance property.
steel structure construction in infrastructural application. However, the present study is only applicable for the selected grade of steel which is used in the present study. So, when we select the different grade of steel
• Deep cryogenic treatment had a positive effect on the corrosion
resistance, whereas, tempering done after deep cryogenic treatment
Fig. 7. SEM images of samples after corrosion a) untreated b) DCT c) DCT + LT. 473
Vacuum 159 (2019) 468–475
S. Ramesh et al.
Fig. 8. Two Dimensional AFM images of all the samples.
Fig. 9. Three dimensional AFM images of all the samples.
Fig. 10. Peak profile analysis of all the samples.
carbide precipitation.
Table 2 Roughness data of all the samples. Sample Details
Rpv
Rq
Ra
untreated DCT DCT + LT
437.695 71.301 94.283
46.483 8.657 15.313
32.888 6.813 12.123
Hence, from the results of the present study, the deep cryogenic treatment of the structural steel samples can be recommended to enhance or improve the corrosion resistance of structural steel for using them in relevant industrial or infrastructural applications. Acknowledgements
• • • • •
showed negative effects. Hence, tempering is not suggested for this material after deep cryogenic treatment. It was also found that the deep cryogenic treatment process for steel material enhances the corrosion resistance of structural steel by 95%. Increase in corrosion resistance in the material was due to uniform and homogenous carbide precipitation in the microstructure of DCT. Hardness and tensile strength test results inferred that there were no negative effects due to deep cryogenic treatment. SEM and AFM studies support towards uniform layer formation and corrosion resistance enhancement of the DCT samples. In the case of DCT + LT samples, due to low temperature tempering process, the system attained microstructural weakness towards corrosion resistance due to slightly non-homogeneous secondary
Authors acknowledge IIT Kanpur, CSIR –SERC and Anna University, Chennai, India for the constant support and encouragement provided for the present study. References [1] Nagaraj M and B.Ravisankar, Enhancing the strength of structural steel through severe plastic deformation based thermomechanical treatment, Mater. Sci. Eng., Available online 25 September 2018. https://doi.org/10.1016/j.msea.2018.09.095. [2] Amin Akhbarizadeh, Kamran Amini, Sirus Javadpour, Effects of applying an external magnetic field during the deep cryogenic heat treatment on the corrosion resistance and wear behavior of 1.2080 tool steel, mater design 41 (2012) 114–123. [3] P. Baldissera, C. Delprete, Deep cryogenic treatment: a bibliographic review, Open Mech. Eng. J. 2 (2008) 1–11. [4] D. Yun, L. Xiaoping, X. Hongshen, Deep cryogenic treatment of high-speed steel and its mechanism, Heat Treat. Metals 3 (1998) 55–59.
474
Vacuum 159 (2019) 468–475
S. Ramesh et al.
[5] S.H. Avner, Introduction to Physical Metallurgy, Tata McGraw-Hill Publishing Co. Ltd., New Delhi, 2003, pp. 305–307. [6] L.H. Vanvlack, Elements of material science and engineering, addison-wesley series in metall, Mater. Eng. 6 (1998) 316–320. [7] D.N. Collins, J. Dormer, Deep cryogenic treatment of a D2 cold-worked tool steel, Heat Treat. Met. 24 (3) (1997) 71–74. [8] Chuang-xian Xiong, Xin-ming Zhang, Yun-lai Deng, Yang Xiao, Zhen-zhen Deng, Buxiang Chen, Effects of cryogenic treatment on mechanical properties of extruded Mg–Gd–Y–Zr (Mn) alloys, J Central South UnivTechnol 14 (2007) 305–309. [9] P. Baldissera, Fatigue scatter reduction through deep cryogenic treatment on the 18NiCrMo5 carburized steel, Mater. Des. 30 (2009) 3636–3642. [10] A. Bensely, D. Senthilkumar, D. MohanLal, G. Nagarajan, A. Rajadurai, Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17, Mater. Char. 58 (2007) 485–491. [11] M. ArockiaJaswin, D. Mohan Lal, Effect of cryogenic treatment on the tensile behaviour of En 52 and 21-4N valve steels at room and elevated temperatures, Mater. Des. 32 (2010) 2429–2437. [12] P. Johan Singh, S.L. Mannan, T. Jayakumar, D.R.G. Achar, Fatigue life extension of notches in AISI 304L weldments using deep cryogenic treatment, Eng. Fail. Anal. 12 (2005) 263–271. [13] S. Zhirafar, A. Rezaeian, M. Pugh, Effect of cryogenic treatment on the mechanical properties of 4340 steel, J. Mater. Process. Technol. 186 (2007) 298–303. [14] Nirmal S. Kalsi, Rakesh Sehgal, Vishal S. Sharma, Cryogenic Treatment of Tool Materials: A Review Materials and Manufacturing Processes 25 (2010) 1077–1100. [15] P. Johan Singh, B. Guha, Fatigue life improvement of AISI 304L cruciform welded joints by cryogenic treatment, Eng. Fail. Anal. 10 (2003) 1–12. [16] P. Baldissera, Fatigue scatter reduction through deep cryogenic treatment on the 18NiCrMo5 carburized steel, Mater. Des. 30 (2009) 3636–3642. [17] H. Hao-huai Liu, J. Wang, H. Yang, B. Shen, Effects of cryogenic treatment on microstructure and abrasion resistance of CrMnB high-chromium cast iron subjected to sub-critical treatment, Mater. Sci. Eng. 478 (2008) 324–328. [18] H. Hao-huai Liu, J. Wang, B. Shen, H. Yang, S. Gao, S. Huang, Effects of deep cryogenic treatment on property of 3Cr13Mo1V1.5 high chromium cast iron, Mater. Des. 28 (2007) 1059–1064. [19] R.F. Barron, R.H. Thompson, Effect of Cryogenic treatment on corrosion resistance, Advances in Cryogenic Engineering Materials 36 (1990) 1375–1379. [20] Z. Zhu, J. Zhao, X. Huang, Effects of cryotreat on the corrosion resistance of the medium melting point castable alloy, West China J. Stomatol. 20 (5) (2002) 316–319. [21] Wang Qiong-Qi, Wang Wei-Ze, Xuan Fu-Zhen, Tu Shan-Tung, A new method improving intergranular corrosion resistance of AISI 304 stainless steel, FM2008 – Evaluation, Inspection and Monitoring of Structural Integrity (2008) 411–416. [22] Aamir Ali, Lei Pan, Lixiang Duan, Zengmin Zheng, Effects of deep cryogenic
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31] [32] [33] [34] [35] [36] [37]
475
treatment, cryogenic and annealing temperatures on mechanical properties and corrosion resistance of AA5083 aluminium alloy, Int. J. Microstruct. Mater. Prop. 11 (2016) 5. Wei Wang, Venkateswaran Srinivasan, Sri Siva, Bensely Albert, Mohan Lal, Akram Alfantazi, Corrosion behavior of deep cryogenically treated AISI 420 and AISI 52100 steel, Corrosion 70 (7) (2014) 708–720. Xiaoyuan Gong, Zhisheng Wu, Fei Zhao, Effect of deep cryogenic treatment on the microstructure and the corrosion resistance of AZ61 magnesium alloy welded joint, Metals 7 (2017) 179. G. Hemath Kumar, H. Mohit, Rajesh Purohit, Effect of deep cryogenic treatment on composite material for automotive Ac system, 5th international conference of materials processing and characterization (ICMPC 2016), Mater. Today: Proceedings 4 (2017) 3501–3505. Yang Gao, Bing-Hui Luo, Zhen-hai Bai, Bin Zhu, Sheng Ouyang, Effects of deep cryogenic treatment on the microstructure and properties of WC—Fe—Ni cemented carbides, Int. J. Refract. Metals Hard Mater. 58 (2016) 42–50. Ilyas Uygura, Husnu Gerengib, Yusuf Arslanc, Mine Kurtayb, The effects of cryogenic treatment on the corrosion of AISI D3 steel, Mater. Res. 18 (3) (2015) 569–574. Y. Cai, Z. Luo &Y. Zeng, Influence of deep cryogenic treatment on the microstructure and properties of AISI304 austenitic stainless steel A-TIG weld, Sci. Technol. Weld. Join. 22 (3) (2017) 236–243. ASTM Standard G3-89, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, Annual book of standards, 2004. ASTM Standard E975-03, Standard Practice for X-Ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation, Annual book of standards, 2004. ASTM Standard E92-82, Standard Test Method for Vickers Hardness of Metallic Materials, Annual book of standards, 2004. ASTM Standard E 8-04, Standard Test Methods for Tension Testing of Metallic Materials, Annual book of standards, 2004. M. Ferhat, A. Benchettara, S.E. Amara, D. Najjar, Corrosion behaviour of Fe-C alloys in a sulfuric medium, J. Mater. Environ. Sci. 5 (4) (2014) 1059–1068. J. Mazur, Investigation on austenite and martensite subjected to very low temperatures, Cryogenics 4 (1964) 36. Mars G. Fontana, Corrosion Engineering, Tata McGraw-Hill, New Delhi, India, 2005. SusheelKalia, Cryogenic processing: a study of materials at low temperatures, J. Low Temp. Phys. 158 (2010) 934–945. H. Yumoto, Y. Nagamine, J. Nagahama, M. Shimotomai, Corrosion and stability of cementite films prepared by electron shower, Vacuum 65 (2002) 527–531.