J. Cent. South Univ. (2015) 22: 3722−3730 DOI: 10.1007/s11771-015-2915-x

Effects of laser heat treatment on salt spray corrosion of 1Cr5Mo heat resistant steel welding joints

KONG De-jun(孔德军)1, 2, GUO Wei(郭卫)1

1. College of Mechanical Engineering, Changzhou University, Changzhou 213164, China;

2. Jiangsu Key Laboratory of Materials Surface Science and Technology (Changzhou University), Changzhou 213164, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2015

Abstract: The surface of 1Cr5Mo heat-resistant steel welding joint was processed with CO2 laser, and the corrosion behaviors before and after laser heat treatment (LHT) were investigated in the salt spray corrosion environments. The microstructures, phases, residual stresses and retained austenite content of 1Cr5Mo steel welding joint before and after LHT were analyzed with optical microscope and X-ray diffraction, respectively. The cracking morphologies and chemical compositions of corrosion products after salt spray corrosion were analyzed with field emission scanning electron microscopy (FESEM) and energy disperse spectroscopy (EDS), respectively, the polarization curves were measured on a PS-268A type electrochemical workstation, and the mechanism of corrosion resistance by LHT was investigated as well. The results show that the passive film of original sample is destroyed owing to the corrosive media penetrating into the subsurface, resulting in the redox reaction. The content of residual austenite in the surface and the self-corrosion potential are increased by LHT, which is contributed to improving the capability of salt spray corrosion resistance. Key words: laser heat treatment; 1Cr5Mo heat-resistant steel; welding joint; salt spray corrosion

1 Introduction

1Cr5Mo heat-resistant steel is generally used as high-temperature pipelines of the thermal cracking and hydrotreating unit in petrochemical industry [1−2], which is frequently used for transporting the materials with high salt content, laying the underground for the restriction of surrounding environments, therefore, the corrosion problems become one of the concerned focuses. During the pipeline laying, welding is generally used to connect pipes, which is a kind of partial heating and melting process, and can cause the structural defects of welded joints, which will develop into the crack source during the corrosion cracking, resulting in the corrosion failure. The corrosion failure of pipeline welding joints is common [3−4], therefore, the capability of welding joint becomes a key factor of affecting service life of the pipeline. The introduction of compressive residual stress and increasing strength by shot peening are the main methods to improve the corrosion resistance currently [5]. But shot peening decreases the surface roughness, and the depth of compressive residual stress is difficult to control, which restricts its industrial applications [6−7]. Laser heat treatment (LHT) is an advanced material

surface treatment technology, which can change the microstructures in the material surface, and bring about the compressive residual stress that can improve the corrosion resistance. Scholars have researched more about the traditional heat treatment and stress corrosion of 1Cr5Mo heat-resistant steel welding joints [8−9], however, the investigations about the resistance of salt spray corrosion by LHT are rarely reported. The surface of 1Cr5Mo steel welded joint was modified by CO2 laser, and the mechanism of LHT on salt spray corrosion resistance of 1Cr5Mo steel welding joints was researched, which would provide the experimental basis of improving corrosion resistance of 1Cr5Mo steel welding joints in the future. 2 Experimental methods

The test material was 1Cr5Mo heat-resistant steel

with chemical compositions (mass fraction, %) as follows: 0.15 C, 0.50 Si, 0.60 Mn, 4.0−6.0 Cr, 0.45− 0.60 Mo, 0.60 Ni, 0.03 P, 0.02 S, and balance Fe. The welding methods were argon arc welding, manual electric arc welding overlaying and double-sided welding. The welding wire type of argon arc welding was H1Cr5Mo, and the chemical compositions are listed in

Foundation item: Project(CXLX14-1098) supported by Jiangsu Province Postgraduate Scientific Research Innovation Program, China Received date: 2014−07−15; Accepted date: 2015−01−31 Corresponding author: KONG De-jun, Associate Professor, PhD; Tel: +86−519−81169812; E-mail: kong-dejun@163.com

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Table 1. The welding electrode type of manual electric arc welding was R507 and the chemical composition are listed in Table 2. Table 1 Chemical composition of H1Cr5Mo wire (mass

fraction, %)

C Mn Si Cr

≤0.12 0.4−0.7 0.15−0.30 4.0−6.0

Mo Ni S P Fe

0.4−0.6 ≤0.30 ≤0.03 ≤0.03 Bal.

Table 2 Chemical composition of R507 electrode deposited

metal (mass fraction, %)

C Cr Mo Mn Si

≤0.12 4.5−6.0 0.4−0.7 0.5−0.9 0.5

P S Cu V Fe

≤0.035 ≤0.03 0.5 0.1−0.35 Bal.

The test samples are divided into two groups: the

original sample and the laser processed sample conducted on a GLS-IB type CO2 laser. The technical parameters are as follows: laser power of 1.5 kW, scanning speed of 15 mm/s, and spot diameter of 4 mm. The morphologies, phases, residual stress and retained austenite content of the samples before and after LHT were analyzed on a D/max 2500PC type X-ray diffractometer, 4XC series microscope and X350-A type X-ray stress analyzer, respectively. The parameters for testing residual stress are as follows: tilt angles Ψ of 0°, 25°, 35° and 45°, starting and termination angle of 163° and 148°, respectively, scanning speed of 2.00 (°)/min, and stress constant of −318 MPa/(°). And the parameters for testing retained austenite content are starting angle of 134°, termination angle of 123°, and scanning step of 0.1°. According to the GB 6468−86 standard, the sample was put in a YQW-250 type salt spray corrosion test chamber with 5% NaCl solution and pH of about 6.5, temperature of (35±1) °C, and spray pressure of 50− 100 kPa. The polarization curves were measured on a PS-268A type electrochemical tester, and the sample size for electrochemical corrosion is 10 mm×10 mm×3 mm, as shown in Fig. 1. The morphologies and compositions of corrosion products were investigated with a JSUPRA55 type FESEM and attached EDS, respectively. 3 Analysis and discussion of experimental

results 3.1 Surface morphologies after corrosion 3.1.1 Morphologies of corrosion products

There are some welding porosities on the surface of

Fig. 1 Sketch of sample for electrochemical corrosion

welded joints before and after LHT. For the sample in the original state, the number of the welding porosities per unit area is more than that of the sample by LHT, and the welding porosities are larger than those of the laser processed sample. As shown in Figs. 2(a) and (b), the welding porosities generally lead to formation of the corrosion pitting, indicating that LHT decreases the quantity and size of the welding porosity to reduce the occurrence of pitting corrosion.

Fig. 2 Morphologies of 1Cr5Mo welded joint before salt spray

corrosion: (a) Original state; (b) After LHT

There are some loose and porous structures and

corrosion cracks on the original welded joint surface, accompanied with a certain amount of exfoliation corrosions. The main cracks run through the whole sample surface, and there are many branch cracks in the vicinity, the width of which is similar to that of the main crack, as shown in Fig. 3(a). After LHT, the corrosion products on the sample surface are more even and compact, the cracks on the surface are not obvious, of

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which the size and number reduce significantly compared with the original sample, and the peeling is also significantly reduced, as shown in Fig. 3(b), indicating that the corrosion resistance of the laser processed sample is better than that of the original sample. The spalled corrosion is attributed to the different electrochemical performances between the welded zone and the base metal, constituting a system of electrochemical corrosion to result in the hydrogen evolution reaction and damage of the corrosion film. The hydrogen evolution reaction mechanism is as follows [5]: 2H2O+2e→H2+2OH− (1) O2+2H2O+4e→4OH− (2) Fe→Fe2++2e (3)

Oxidation-reduction reaction: Fe→2e+Fe2+ (4)

Hydrolysis reaction: Fe2++H2O→Fe(OH)2+H+ (5) H++Cl−→HCl (6)

Fig. 3 Morphologies of sample surface after salt spray

corrosion: (a) Original state; (b) After LHT

3.1.2 Corrosion pits

The welded porosities and sulfur impurities lead to the corrosion pittings, and the shape of original sample looks like a jujube covered by the cracks on the bottom, as shown in Fig. 4(a). The dimensions of corrosion pitting by LHT are significantly smaller than those of the original sample, and no cracks exist at the pitting bottoms, as shown in Fig. 4(b). The main reasons for

these changes are as follows: 1) LHT not only refines the surface grains but also refines the defect sizes, thereby, the size of corrosion pitting is reduced; 2) The corrosive medium easily penetrates into the matrix, leading to oxidation−reduction reaction, and the surface of original sample exhibits loose and porous structures. Moreover, the products of oxidation−reduction reaction would produce hydrolysis reaction and increase the concentration of H+. In order to maintain the charge balance, Cl− in the medium penetrates through the corrosion layer and gets into the corrosion pittings to form HCl acid solution, which causes more damages of the corrosion pitting, indicating that LHT reduces the size of corrosion pittings. The reaction mechanism is as follows [10].

The oxidation−reduction reaction: Fe→2e+Fe2+ (4)

Hydrolysis reaction: Fe2++H2O→Fe(OH)2+H+ (5) H++Cl−→HCl (6)

Fig. 4 Morphologies of corrosion pitting after salt spray

corrosion: (a) Original state; (b) After LHT

3.1.3 Corrosion cracks

There is a deflection phenomenon of the main crack propagation path on the original sample surface, of which some dendritic branch cracks also appear. The width of the main crack is about 0.5 μm, while the branch crack width is smaller than the main crack, as shown in Fig. 5(a). The crack propagation path generally appears at the position, where the structure is relatively

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weaker in the internal material. The crack propagation deflection of the laser processed sample occurs at points A and B, and there is dendritic branch crack at point A, of which the main crack width is about 0.2 μm, as shown in Fig. 5(b). During the crack propagation, the crack deflection would consume some energies to form deflection toughening [11], resulting in improving the material strength and further hindering the crack expansion. The crack depth is proportional to the crack width, and consequently, the crack depth of the laser processed sample is less than the original sample, which shows that LHT holds back the production of cracks in the surface of 1Cr5Mo heat-resistant steel and improves the ability of corrosion crack closure.

Fig. 5 Surface crack morphologies after salt spray corrosion:

(a) Original state; (b) After LHT

3.2 EDS analysis

The chemical composition of the welded joint surface of the original sample after salt spray corrosion is as follows (mass fraction, %): 43.44 C, 47.51 O, 0.19 Al, 0.44 Si, 0.23 Cl, 0.16 Ca, 0.16 Cr and 7.86 Fe, as shown in Fig. 6(a). And the chemical composition of the welded joint surface by LHT after corrosion is as follows: 46.28 C, 44.83 O, 0.13 Al, 0.29 Si, 0.13 S, 0.36 Cl, 0.18 Ca, 0.76 Cr and 7.04 Fe, as shown in Fig. 6(b). The contents of other chemical elements are too low to be analyzed, and the elements of Al and Ca are impurities of the test chamber. After LHT, the element S appears on the surface because of the oxidizing reaction of Fe and S during LHT, which acts as the trigger of corrosion pitting sources. The corrosion products are mainly composed of Fe, Cr, Cl and O, therefore, the molecular formula of

corrosion products could be considered as MexOyHzCln [5]. After LHT, the oxygen content on the surface decreases from 47.51% to 44.83%, and the Fe content reduces from 7.86% to 7.04%, indicating that the corrosion products are less than those of the original sample. The content of Cr increases by 2.3 times, resulting in forming passive film that prevents the corrosion medium from penetrating into the inner surface, so that the element Cl increases from 0.23% to 0.36% on the surface. The element Si reduces from 0.44% to 0.29% because of element diffusion outward during LHT, however, the activity of Si is weaker than that of other elements, resulting in the content reduction on the surface, showing that the content of element Cr on the surface increases after LHT, which is beneficial to improving the corrosion resistance of the material.

Fig. 6 EDS analysis after salt spray corrosion: (a) Original state;

(b) After LHT

3.3 XRD analysis

The XRD patterns of the samples before and after LHT are shown in Fig. 7. The main phases of the original sample are Fe and CrC, of which the CrC phase is a stable compound that would influence the activity of Cr and reduce the corrosion resistance. After LHT, the phases include FeC, Fe-Cr, NiO, FeS2, Fe3O4, MnO2 and

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MoC, in which FeC is a gap compound formed by Fe and C in the surface during LTH; Fe3O4, MnO2 and FeS are the products of Fe and O, Mn and O, Fe and S, respectively, which are proved to be the source of corrosion pitting. MoC is the product of Mo of the welded joint reacted with C at high temperature, which has a good corrosion resistance. The NiO and SiO2 are dense oxides that could improve the corrosion resistance. In addition, Mo and Cr have the complex effect that could improve the self-repair ability of the passive film to maintain the stability. The Fe-Cr of the laser processed sample could improve the corrosion resistance, because the electrode potential of Cr is lower than that of Fe. Cr could be corroded by anodic dissolution to form a dense corrosion product film that protects the matrix during the corrosion. The protective mechanism is as follows [12].

Cathodic reaction: 2H2O+2e→H2+2OH− (7) O2+2H2O+4e→4OH− (8)

Anodic reaction: Cr3++3OH−→Cr(OH)3+3e (9)

Fig. 7 XRD analysis of welded joints before and after LHT

3.4 EDS analysis of interface after corrosion

The scanning position of original sample interface is shown in Fig. 8(a). The content of Fe in the corrosion layer performs as the gradient shape, and there is a peak between the two layers, as shown in Fig. 8(b). The corrosive medium passes through the surface film into the matrix to continue the corrosion reaction. The content of Cr in the corrosion layer is about 0.1 times that of Fe, which is less than that in the matrix, as shown in Fig. 8(c). The contents of Mo and Mn in the surface layer are also obviously reduced, as shown in Figs. 8(d) and (e). This is because the surface corrosion products are loose and prone to peeling. The content of Si relatively increases since the other elements are spalled off, as shown in Fig. 8(f), indicating that the Si element is stable

in the environment of salt spray corrosion. The scanning position of the laser processed sample

interface is shown in Fig. 9(a). The contents of Fe and Cr in the corrosion layer change little after LHT, as shown in Figs. 9(b) and (c), indicating that there is rarely corrosion peeling phenomenon during the corrosion. The contents of Mn and Mo in the surface layer are almost zero, as shown in Figs. 9(d) and (e), indicating that Cr is the main element to improve the corrosion resistance. At the depth of 60−65 μm, the Cr and Mn contents increase sharply, as shown in Figs. 9(c) and (e). This is because LHT refines the grain and produces a large number of grain boundaries to provide more channels for the elements to diffuse into the surface layer. The activity of Si is weaker than that of other elements that are not easily diffused, as shown in Fig.9 (f), showing that the corrosion film is compact after LHT to reduce the peeling of corrosion products. 3.5 Analysis of polarization curves

The polarization curves of the samples before and after LHT in 5% NaCl solution are shown in Fig. 10. After LHT, the polarization curve shifts to the left, and the curve is steeper than that of the original sample, indicating that the electrode process is more difficult than that of the original sample. In addition, the self-corrosion potential is higher than the original sample, which results in improving the ability of corrosion resistance. The smaller the potential is, the easier it turns as the anode and dissolves. From the viewpoint of the anode, there is a reciprocating passive region of the original sample formed by dissolving the passive film continuously and repairing itself constantly at the same time. In the transpassive region, the current density increases along with the rising potential and reaches a pitting inflection Eb. It is the Cl− of the corrosive medium that destroys the passive film, gets into the matrix and produces the continuous reaction. The current density of the laser processed sample increases rapidly with the potential rising in the beginning stage and stabilizes in the passive region, during which the dense and compact corrosion products form on the sample surface after LHT, preventing the corrosion medium from penetrating into the inner surface. The current of the passive region is small, indicating that LHT improves the long-term corrosion resistance on the material surface. 3.6 Analysis and discussion 3.6.1 Analysis of surface metallographies

The metallographic structures of welded joint in original state are shown in Fig. 11(a). After LHT, the quantity of grain per unit area increases and arranges more closely, as shown in Fig. 11(b). The coarse grain

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Fig. 8 Line scan results of original sample interface after salt spray corrosion: (a) Scanning position; (b) Fe; (c) Cr; (d) Mo; (e) Mn;

(f) Si

boundaries are easily enriched by the inclusions such as MnS [13], which are often the pitting sources that cause local damage to the passive film. The damaged corrosion film is formed as the anode and the undestroyed area as the cathode, which could constitute a corrosion cell and generate electrochemical reaction to accelerate the forming of corrosion pittings. In addition, the sample microstructures after LHT are even and close, which leads to the more compact and stronger self-repair

capable passive film formed than that of the original sample [14], therefore, the corrosive medium does not easily pass through the passive film. 3.6.2 Analysis of residual austenite

The contents of residual austenite and martensite of welded joint in original state are respectively 5.8% and 94.2%, as shown in Fig. 12(a). After LHT, the content of residual austenite increases to 10.5%, and that of the martensite reduces to 89.5%, as shown in Fig. 12(b). The

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Fig. 9 Line scan results of laser processed sample interface after salt spray corrosion: (a) Scanning position; (b) Fe; (c) Cr; (d) Mo; (e)

Mn; (f) Si

different microstructures have different electrode potentials. The potential of martensitic phase is lower than that of the austenite, which induces martensite phase to act as anode and austenite as cathode, as a result, the martensite phase is selectively dissolved and the passive film on the welded joint surface is uneven. The Cl− of the corrosive medium preferentially adsorbs on the position of the passive film, which is relatively weak, resulting in

accelerating the dissolution rate of the position and destroying the stability of the passive film. Moreover, the martensite phase gathers with high density of dislocated structures, and the phase boundary energy is relatively high, which would easily produce a pitting source in the solution with Cl− to enhance the pitting sensitivity. The martensite content in the welded joint surface decreases after LHT, which improves the stability of the passive

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Fig. 10 Polarization curves in corrosive environments before

and after LTH

film and reduces the forming of pitting source. 3.6.3 Analysis of residual stress

The residual stresses on the welded joints before and after LHT are shown in Fig. 13, decreasing from (−234±31) MPa (Fig. 13(a)) in original state to (−519± 29) MPa (Fig. 13(b)) after LHT. When laser acts on the welded joint surface, the energy reduces gradually in the depth direction, resulting in uneven heating of the sample. The particle has generated plastic deformation, and could not return to the original balance position, as a result, the compressive residual stress has been improved [15]. The compressive residual stress could increase the threshold of the crack propagation, and the intensity factor of failure stress delays the crack incubation time, forming the crack closure effect [16]. The compressive residual

Fig. 11 Metallographic structures of welded joints: (a) Original state; (b) After LHT

Fig. 12 Analysis of residual austenite and martensite contents: (a) Original state; (b) After LHT

Fig. 13 Analysis of residual stress of samples before and after LHT: (a) Original state at residual stress of (−234±31) MPa; (b) After

LHT at residual stress of (−519±29) MPa

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stress is improved by LHT, which would be conducive to hinder the surface cracks from expanding. 4 Conclusions

1) The passive film of 1Cr5Mo steel welded joint in original state is destroyed after salt spray corrosion, and the corrosion media are penetrated into the inner layer and continue corrosion reaction.

2) The polarization curve of welded joint after LHT maintains at the passive region, which can prolong the service life of the welded joint in the corrosive environment.

3) The self-corrosion potential and the content of residual austenite on the surface of 1Cr5Mo steel welded joint are increased by LHT, which is the main mechanism of improving corrosion resistance. References [1] ZOU Yang, PAN Chun-xu, FU Qiang, CHEN Chao. In situ

observations for corrosion process at fusion boundary of Cr5Mo

dissimilar steel welded joints in H2S containing solution [J]. Acta

Metallurgica Sinica, 2005, 41(4): 421−426.

[2] ZHAO J, HAN S Q, XIE S M, GAO H B, WANG L. Remaining life

assessment of a 1Cr5Mo steel by using Z parameter method [J]. Acta

Metallurgica Sinica: English Letter, 2004, 17(4): 601−605.

[3] JIANG Wei, KADDA Y. Effect of welding sequence on residual

stress distribution in a multipass welded piping branch junction [J].

International Journal of Pressure Vessels and Piping, 2012, 95(6):

39−47.

[4] KUNIO O, HIROYUKI N, HIROTO I. Development of probabilistic

fracture mechanics analysis codes for reactor pressure vessels and

piping considering welding residual stress [J]. International Journal

of Pressure Vessels and Piping, 2010, 87(6): 2−10.

[5] WU Yong-zhong, KONG De-jun, LONG Dan, FU Gui-zhong.

Effects of laser shock wave on salt spray corrosion of X70 pipeline

steel welded lines [J]. Transactions of the China Welding Institution,

2012, 33(12): 101−104. (in Chinese)

[6] SHARIFITABAR M, HALVAEE A, KHORSHAHIAN S.

Microstructure and mechanical properties of resistance upset butt

welded 304 austenitic stainless steel joints [J]. Materials and Design,

2011, 32: 3854−3864.

[7] ZHANG Guo-dong, ZHOU Chang-yu. Effects of welding residual

stress on creep of welded joint [J]. Transactions of the China Welding

Institution, 2007, 28(8): 99−102. (in Chinese)

[8] WANG Xiao-hong, SHI Yi-jun, SHAO Chun-yu, XUE Wei, WEN

Ting. Effect of high sulfur and high acid value crude oil temperature

on corrosion resistance of 1Cr5Mo steel [J]. Transactions of

Materials and Heat Treatment, 2012, 34(10): 160−164. (in Chinese)

[9] CHEN Song. Research status of high temperature stress corrosion

and protective methods for 1Cr5Mo steel [J]. Hot Working

Technology, 2013, 42(12): 29−32. (in Chinese)

[10] LIANG Jin-ming, TANG Di, WU Hui-bin, ZHANG Peng-cheng,

LIU Wei, MAO Hong-yan, LIU Xia-tong. Corrosion behaviors of Cr

low-alloy steel in bottom plate environment of cargo oil tanks [J].

Journal of South China University of Technology: (Natural Science),

2013, 41(10): 72−98. (in Chinese)

[11] LIANG Jun, WANG Chao, LUAN Xu, WANG Bao-lai. Analysis on

toughening mechanisms of different sic particle reinforced thermal

protective ceramics [J]. Rare Metal Materials and Engineering, 2009,

38(s2): 894−897. (in Chinese)

[12] SUN Jian-bo, LIU Wei, CHANG Wei, ZHANG Zhong-hua, LI

Zhong-tao, YU Tian, LU Min-xu. Characteristics and formation

mechanism of corrosion scales on low-chromium X65 steel in CO2

environment [J]. Acta Metallurgica Sinica, 2009, 45(1): 84−90. (in

Chinese)

[13] SHI Jin-jie, SUN Wei, GENG Guo-qing. Corrosion resistances of

passive films on low-carbon rebar and fine-grained rebar in alkaline

media [J]. Acta Metallurgica Sinica, 2011, 47(4): 449−454. (in

Chinese)

[14] ZHANG Jun-wang, WANG Wen-xian, HUANG Yan-ping, WANG

Bao-dong, LIU Xu. Electrochemical corrosion properties for weld

metal of austenitic stainless steel [J]. Transactions of the China

Welding Institution, 2007, 28(2): 103−107. (in Chinese)

[15] DU Cui-wei, Liu Zhi-yong, LIANG Ping, JIA Gai-feng, LI

Xiao-gang. Short-term corrosion behavior of X70 pipeline steel with

different microstructure in Ku’erle soil with saturated water [J]. Heat

Treatment of Metals, 2008, 33(6): 80−84. (in Chinese)

[16] ZHANG Xing-quan, ZHANG Yong-kang, ZHOU Jian-zhong.

Experimental research on narrow strip laser peening forming [J].

Chinese Journal of Lasers, 2007, 34(10): 1446−1450. (in Chinese)

(Edited by FANG Jing-hua)