metals Article
Effects of Small Ni Addition on the Microstructure and Toughness of Coarse-Grained Heat-Affected Zone of High-Strength Low-Alloy Steel Gang Huang 1 , Xiangliang Wan 1, *, Kaiming Wu 1, *, Huazhong Zhao 2 and Raja Devesh Kumarmr Misra 3 1
2 3
*
The State Key Laboratory of Refractories and Metallurgy, Hubei Province Key Laboratory of Systems Science in Metallurgical Process, International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, China;
[email protected] Wuhan Anchor Welding consumables Co., Ltd., Wuhan 430081, China;
[email protected] Department of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79968, USA;
[email protected] Correspondence:
[email protected] (X.W.);
[email protected] (K.W.); Tel.: +86-27-68862772 (X.W. & K.W.); Fax: +86-27-68862606 (X.W. & K.W.)
Received: 14 August 2018; Accepted: 11 September 2018; Published: 13 September 2018
Abstract: The objective of the present study is to investigate the effects of nickel (Ni) on the microstructure and impact toughness of coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steel. It was observed that the microstructure of CGHAZ predominantly consisted of bainite and a small proportion of martensite-austenite (M-A) constituents and acicular ferrite (AF). With increased Ni content, the percentage of M-A constituent decreased and AF increased; consequently, the impact toughness of CGHAZ increased. The study revealed that a small addition of nickel significantly affected the formation of M-A constituents and AF; however, no obvious influence was observed on the bainitic microstructure of high-strength low-alloy steel. Keywords: high-strength low-alloy steel; Ni; coarse-grained heat-affected zone; microstructure; impact toughness
1. Introduction High-strength low alloy steels (HSLA) are widely used in applications requiring welding because of their high toughness, high strength, cold formability, and good weldability. In recent years, several studies have been carried out using high heat input welding to reduce cost and improve welding efficiency. However, the high heat input welding thermal cycle results in a coarse-grained heat-affected zone (CGHAZ), as well as degradation in the toughness of steels [1]. Therefore, improvement in the toughness of HAZ is an important aspect when high heat input is used. The toughness of HAZ is strongly influenced by the microstructural features of steels [2]. Coarse bainite in the HAZ of HSLA steel contains parallel bainitic ferrite and martensite-austenite (M-A) constituents [3]. Thus, it is difficult to obtain superior HAZ toughness because of the hard M-A constituents and the low-angle grain boundary between parallel bainitic ferrite, which facilitates nucleation and propagation of cracks. Previous studies have revealed that the microstructural features of HAZ greatly depend on the alloying elements in HSLA steels [4]. Deoxidizing alloying elements, such as titanium and zirconium, provide favorable nucleation sites at grain boundaries for AF during the welding thermal cycle [5]. Some studies have also proposed that niobium, aluminum, and silicon can modify the morphology of M-A constituents [6–8].
Metals 2018, 8, 718; doi:10.3390/met8090718
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Nickel solid solution solution strengthening strengthening element Nickel (Ni) (Ni) is is generally generally present present as as aa solid element in in ferrite ferrite and and increases increases the strength of steel by solid solution strengthening. Ni also has a strong influence on the toughness of the strength of steel by solid solution strengthening. Ni also has a strong influence on the toughness weld metals. Previous studies have indicated that with increasing Ni Ni content, thethe microstructure of of weld metals. Previous studies have indicated that with increasing content, microstructure the weld metal can be refined [9]. However, when Ni content exceeds a certain value, the toughness of the weld metal can be refined [9]. However, when Ni content exceeds a certain value, the toughness of of the the weld weld metal metal is is reduced reduced because because of of the the formation formation of of martensite martensite [10]. [10]. Superior Superior low-temperature low-temperature toughness austenite, which absorbs high energy by toughness in in high-Ni high-Nisteel steelcan canbe beobtained obtaineddue duetotothe theretained retained austenite, which absorbs high energy transformation of the retained austenite to strain-induced martensite during Charpy impact testing [11]. by transformation of the retained austenite to strain-induced martensite during Charpy impact However, an However, understanding of the effect of Ni on microstructural and impact toughness testing [11]. an understanding of the effect of Ni ontransformation microstructural transformation and of CGHAZ of HSLA steels is relatively limited and requires in-depth study. impact toughness of CGHAZ of HSLA steels is relatively limited and requires in-depth study. In In the the present present study, study, the the effect effect of of Ni Ni on on microstructural microstructural transformation transformation during during the the welding welding thermal thermal cycles cycles of of HSLA HSLA steels steels was was studied, studied, and and the the underlying underlying toughening toughening mechanism mechanism of of CGHAZ CGHAZ was was also also explored. explored. 2. Experimental Procedure 2. Experimental Procedure Three Ni, and and 0.43% 0.43% Ni Ni steel, were prepared Three steel steel samples, samples, Ni-free, Ni-free, 0.23% 0.23% Ni, steel, were prepared in in aa 10 10 kg kg vacuum vacuum melt induction furnace (Jinzhou North Electric Furnace Factory, Jinzhou, Liaoning, China) (Table 1). 1). melt induction furnace (Jinzhou North Electric Furnace Factory, Jinzhou, Liaoning, China) (Table ◦ The C and The cylindrical cylindrical ingots, ingots, 120 120 mm mm in in diameter diameterand and100 100mm mmin inlength, length,were werereheated reheatedtoto1250 1250±± 20 20 °C and forged into rectangular billets with cross-sections 30 mm × 30 mm. The samples were then machined forged into rectangular billets with cross-sections 30 mm × 30 mm. The samples were then machined into 11 mm 100 mm. mm. The The simulation simulation of of CGHAZ CGHAZ was was carried carried out out using into dimensions dimensions of of 11 11 mm mm × × 11 mm × × 100 using aa Gleeble Gleeble 3800 3800 (Dynamic (Dynamic Systems Systems Inc., Inc., Austin, Austin, TX, TX, USA). USA). A A schematic schematic diagram diagram of of simulated simulated thermal thermal cycles is presented in Figure 1.The peak temperature, heating rate, and holding time cycles is presented in Figure 1.The peak temperature, heating rate, and holding time for for thermal thermal cycle cycle ◦ C, 300 ◦ C/s, and 3 s, respectively. The cooling times from 800 ◦ C to 500 ◦ C (t simulation were 1350 ) simulation were 1350 °C, 300 °C/s, and 3 s, respectively. The cooling times from 800 °C to 500 °C (t8/5 8/5) were were 10.6 10.6 ss and and 52.8 52.8 s, s, respectively, respectively,which whichwere wereapproximately approximatelyequivalent equivalentto toheat heatinputs inputsof of20 20kJ/cm kJ/cm and 100 kJ/cm, respectively [12,13]. and 100 kJ/cm, respectively [12,13]. Table 1. The chemical composition of the investigated steels (wt.%). Table 1. The chemical composition of the investigated steels (wt.%). Sample Sample Ni-free Ni-free 0.23%Ni Ni 0.23% 0.43% Ni 0.43% Ni
CC 0.054 0.054 0.056 0.056 0.052 0.052
SiSi 0.19 0.19 0.20 0.20 0.22 0.22
Mn Mn 1.56 1.56 1.58 1.58 1.55 1.55
Nb Nb 0.038 0.038 0.036 0.036 0.037 0.037
Ni Ni -0.23 0.23 0.43 0.43
Ti Ti 0.012 0.012 0.015 0.015 0.014 0.014
Mo Mo 0.020 0.020 0.019 0.019 0.018 0.018
Al Al 0.022 0.022 0.025 0.025 0.024 0.024
Fe Fe Bal. Bal. Bal. Bal. Bal. Bal.
Figure Schematic illustration illustration of Figure 1. 1. Schematic of simulated simulated thermal thermal cycles cycles of of the the CGHAZ. CGHAZ.
After the simulation, standard Charpy v-notch specimens of dimensions 10 mm × 10 mm × 55 mm were machined for Charpy impact test at −20 ◦ C. The microstructure and fracture surface of the
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After the simulation, standard Charpy v-notch specimens of dimensions 10 mm × 10 mm × 55 mm were machined for Charpy impact test at −20 °C. The microstructure and fracture surface of the samples were optical microscope (OM,(OM, Olympus Corporation, Tokyo, Japan.) scanning samples werestudied studiedbyby optical microscope Olympus Corporation, Tokyo, and Japan.) and electron microscope (SEM, FEI, Boston, MA, USA). The crystallographic grain size and the grain scanning electron microscope (SEM, FEI, Boston, MA, USA). The crystallographic grain size and the boundary misorientation were analyzed by electron backscattered diffraction (EBSD, EDAX, New grain boundary misorientation were analyzed by electron backscattered diffraction (EBSD, EDAX, York, York, NJ, USA) The M-A and AF and grains the digital images of images simulated New NJ, technique. USA) technique. Theconstituents M-A constituents AFingrains in the digital of CGHAZ were selected and painted black using Adobe Photoshop (version 6.0, San, Jose, simulated CGHAZ were selected and painted black using Adobe Photoshop (version 6.0, CA, San, USA) Jose, (Figure 2). (Figure The length, width, density, and area percentage M-A constituents and AF grains CA, USA) 2). The length, width, density, and area of percentage of M-A constituents andwere AF estimated using Image-Pro Plus (version 6.0, MEDIA, Rockville, MD, USA). Thirty digital images were grains were estimated using Image-Pro Plus (version 6.0, MEDIA, Rockville, MD, USA). Thirty digital analyzed to obtain an average of the above images were analyzed to obtainvalue an average value parameters of the above[12,13]. parameters [12,13].
Figure Figure 2. 2.(a) (a)Optical Opticaland and(c) (c)SEM SEMimages imagesshowing showingrepresentative representativeacicular acicularferrite ferriteand andM-A M-Aconstituents, constituents, respectively, inthe thesimulated simulatedCGHAZ CGHAZ Ni-free steel kJ/cm welding thermal respectively, in of of Ni-free steel withwith 100 100 kJ/cm heatheat inputinput welding thermal cycle. cycle. (b,d) are processed ofrespectively, (a,c), respectively, using Adobe Photoshop. (b,d) are processed imagesimages of (a,c), using Adobe Photoshop.
3. Results Results 3. 3.1. Microstructural Characteristics of Simulated CGHAZ 3.1.Microstructural Characteristics of Simulated CGHAZ Figure 3 shows the optical micrographs of simulated CGHAZ for three steel samples. It was Figure 3 shows the optical micrographs of simulated CGHAZ for three steel samples. It was observed that the prior austenite grains became coarse in all the samples. The microstructure of all observed that the prior austenite grains became coarse in all the samples. The microstructure of all the samples predominantly consisted of bainite and a small percentage of AF and M-A constituents. the samples predominantly consisted of bainite and a small percentage of AF and M-A constituents. The AF grains were scattered among prior austenite grains, and some of them were also associated The AF grains were scattered among prior austenite grains, and some of them were also associated with inclusions. The bainitic ferrite sheaves nucleated at the grain boundaries and grew into the with inclusions. The bainitic ferrite sheaves nucleated at the grain boundaries and grew into the interior of the grain. Furthermore, the M-A constituents were observed in bainite packets, as well as interior of the grain. Furthermore, the M-A constituents were observed in bainite packets, as well as between the boundaries of bainite packets. between the boundaries of bainite packets.
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Figure 3. 3. Optical micrographs of of simulated CGHAZ in in specimens (a,b) Ni-free, (c,d) 0.23% Ni,Ni, and Figure Optical micrographs simulated CGHAZ specimens (a,b) Ni-free, (c,d) 0.23% and (e,f) 0.43% Ni,with heat input of (a,c,e) 20 kJ/cm, and (b,d,f) 100 kJ/cm, respectively. (e,f) 0.43% Ni, with heat input of (a,c,e) 20 kJ/cm, and (b,d,f) 100 kJ/cm, respectively.
3.2. Quantitative Analysis of Microstructure Simulated CGHAZ 3.2. Quantitative Analysis of Microstructure of of Simulated CGHAZ Prior austenite grain size is crucial microstructural evolution [14]. The size prior austenite Prior austenite grain size is crucial forfor microstructural evolution [14]. The size of of prior austenite grains was measured averaging thelong longand andshort shortaxes axesofofthe thegrains. grains.The The mean equivalent grains was measured bybyaveraging the mean equivalent diameters and standard deviation prior austenite grains (taken from about optical micrographs diameters and standard deviation of of prior austenite grains (taken from about 2020 optical micrographs 500magnification) × magnification) presented in Figure The equivalent diameters prior austenite grains at at 500× areare presented in Figure 4a.4a. The equivalent diameters of of prior austenite grains were therange rangeof of 55–60 55–60 µm and increased to 70–80 µm for were ininthe μm at at20 20kJ/cm kJ/cmheat heatinput inputwelding, welding, and increased to 70–80 μm100 forkJ/cm. 100 However, no significant change was noticed with with increased Ni content (Figure 4a). 4a). kJ/cm. However, no significant change was noticed increased Ni content (Figure
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Figure 4. (a) Prior austenite grain size and (b) mean length, (c) width, and (d) area percentage of Figure Prior austenite austenitegrain grainsize sizeand and(b) (b)mean mean length, width, percentage Figure 4. 4. (a) (a) Prior length, (c) (c) width, andand (d) (d) areaarea percentage of of acicular ferrite in samples subjected to different heat inputs. acicular ferrite ferrite in in samples acicular samples subjected subjectedtotodifferent differentheat heatinputs. inputs.
AFAF is one of of the most important constituents for improving the toughness of CGHAZ CGHAZ HSLA isis one most important constituents for improving toughness of CGHAZ of HSLA AF one of the the most important constituents for improving thethe toughness of ofofHSLA steels [3].[3]. TheThe mean length, width, and area percentage of was measured (Figure 4b–d). TheThe mean steels mean length, width, and area percentage of was measured (Figure 4b–d). mean steels [3]. The length, width, and area percentage ofAF AFAF was measured (Figure 4b–d). The mean length and width of AF increased with increased Ni content (Figure 4b,c). The area percentage of AF length and width of AF increased with increased Ni content (Figure 4b,c). The area percentage of length and width AF increased with increased Ni content (Figure 4b,c). The area percentage of AF AF also increased with increased NiNi content and heat input (Figure 4d). also increased with increased Ni content and heat input (Figure 4d). also increased with increased content and heat input (Figure 4d). Figure 5 shows thethe SEM images ofofof block-like and film-like M-A constituents inthe thethe simulated Figure shows SEM images block-like and film-like M-A constituents in simulated Figure 55 shows the SEM images block-like and film-like M-A constituents in simulated CGHAZ. It is noted that the size, density, and area percentage of M-A constituents varied with CGHAZ. It is noted that the size, density, and area percentage of constituents varied with CGHAZ. It is noted that the size, density, and area percentage of M-A constituents varied with increased Ni content and heat input (Figure 6). The mean length of M-A constituents decreased with increased Ni Ni content andand heatheat input (Figure 6). The lengthlength of M-Aofconstituents decreased with increased content input (Figure 6).mean The mean M-A constituents decreased increased content (Figure 6a);however, however, thedensity density of M-A M-A constituents did not change with increased Ni Ni content (Figure 6a); the of constituents did did notnot change with with increased Ni content (Figure 6a); however, the density of M-A constituents change with increased Ni content (Figure 6b). It was observed that the area percentage of M-A constituents also increased Ni content (Figure 6b). It was observed that the area percentage of M-A constituents also increased Ni content (Figure 6b). It was observed that the area percentage of M-A constituents also decreased with increased Ni content(Figure (Figure6c). 6c).Previous Previousstudies studiesreported reportedthat that M-A constituents decreased with increased Ni content Previous studies reported thatM-A M-A constituents decreased with increased Ni content (Figure 6c). constituents with with large length (>2 μm and small length/width ratio (2 µmμm and small length/width ratio (
