effect of normalizing temperature on microstructure and mechanical properties of a nb v microalloyed large forging steel

NHỮNG BÀI BÁO NGHIÊN CỨU VỀ VẬT LIỆU ĐANG ĐƯỢC PHỔ BIẾN RỘNG RÃI. GIÚP CÁC BẠN CÓ THÊM THÔNG TIN CHÍNH XÁC VỀ MỘT SỐ NGHIÊN CỨU VẬT LIỆU CƠ KHÍ

Author’s Accepted Manuscript

microstructure and mechanical properties of a Nb-V microalloyed large forging steel

Xin-li Wen, Zhen Mei, Bo Jiang, Li-chong Zhang, Ya-zheng Liu

To appear in:Materials Science & Engineering A

Received date: 26 May 2016 Revised date:19 June 2016 Accepted date: 20 June 2016

Cite this article as: Xin-li Wen, Zhen Mei, Bo Jiang, Li-chong Zhang and Ya-zheng Liu, Effect of normalizing temperature on microstructure and mechanical properties of a Nb-V microalloyed large forging steel, Materials Science &Engineering A, http://dx.doi.org/10.1016/j.msea.2016.06.059

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Effect of normalizing temperature on microstructure and mechanical properties of a Nb-V microalloyed large forging steel

Xin-li Wen, Zhen Mei, Bo Jiang, Li-chong Zhang, Ya-zheng Liu*

School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Corresponding author lyzh@ustb.edu.cn

Abstract

The microstructure of a microalloyed large forging steel with different normalizing temperatures ranging from 820 ºC to 940 ºC were characterized The evolution of austenite formation was determined in a range of heating temperature from 730 ºC to 940 ºC The mechanical properties were evaluated by tensile test and Charpy V-notch impact test The relationship between the microstructure and the properties was discussed The results indicated that the microstructure composed of fine-grained layers (FGL) and coarse-grained layers (CGL) was obtained at 820 ºC The finest and most homogeneous microstructure and optimal comprehensive mechanical properties were obtained at the normalizing temperature 880 ºC There was a Hall-Petch relationship between the yield strength

and the average grain size, and a linear relationship between the impact energy(KV2) and the reciprocal of the

square root of the grain size (D-1/2) Both the strength and toughness of the steel can be attributed to grain refinement

Keywords: Normalizing temperature, Microstructure, Mechanical properties, Nb-V microalloyed, Large forging

steel, grain refinement

1 Introduction

Microalloyed large forging steel has been widely used for engineering components which require high strength, good toughness and large size Large forging steel is usually made from large casting ingots The as-forged microstructure of the steel is generally composed of coarse ferrite and banded pearlite which consequently lead to limited impact toughness Due to the characteristic of forging process and the large volume of forging, conventional technology such as thermo-mechanical treatment or accelerated cooling is infeasible for grain refinement in order to improve toughness

However, it is likely to increase toughness levels of microalloyed large forging steel through grain refinement by normalizing Zhao et al [1] studied the effect of W addition and normalizing conditions on microstructure and mechanical properties of microalloyed forging steels, four kinds of microalloyed forging steels were produced by varying W additions(0, 0.5, 1 and 2 wt%), heat treatment was carried out at temperatures ranging from 840 ºC to 950 ºC followed by air and furnace cooling, the results showed that the microstructure and mechanical properties of the microalloyed forging steels were closely related to the W content, normalizing temperature and cooling method after normalizing Zhao et al [2] studied the effect of hot forging, normalizing temperature (840 ºC-950 ºC) and cooling method (air and furnace cooling) after normalizing on the toughness and tensile properties of a microalloyed cast steel, the results showed that remarkable improvement in toughness and tensile properties can be obtained by hot forging, proper normalizing temperature and air cooling after normalizing Zhao et al [3] studied the effect of normalizing temperature (950 ºC-1200 ºC) and cooling method (furnace, air and water cooling) after normalizing on the toughness and tensile properties of a low-carbon microalloyed cast steel, the results showed that heat treatment at 1100 ºC for 2 hours followed by furnace cooling leaded

to the best combination of excellent Charpy impact and tensile properties

The above research focused on the low carbon Nb-Ti microalloyed steel, there was no many studies on Nb-V microalloyed steel in which the V content can be as much as 0.1 wt% Besides, the sizes of samples used by Zhao et al [1-3] were 11 ×11 × 60 mm3 or 11 ×11 × 110 mm3, the air cooling rate was above 3ºC/s There were rare studies on large section forging steel with the diameter larger than 250 mm and the air cooling rate below 0.05ºC/s Austenite formation in low carbon steels has been studied extensively in the literature starting from different microstructures [4-6] Previous work has shown that in ferritic-pearlitic microstructures the formation of austenite was described as taking place in three main successive steps: (1) nucleation of austenite in pearlite colonies, ferrite-pearlite grain boundaries or ferrite-ferrite grain boundaries, (2) rapid growth of austenite consuming pearlite, (3) slower growth of austenite consuming ferrite [7,8] Based on the above theory, as for ferrite-pearlite banded microstructure in large forging steel, austenization in ferrite bands and pearlite bands are asynchronous Nonetheless, rare literature has studied this phenomenon What’s more, there were no many studies on the effect of intercritical normalizing on microstructure and mechanical properties of Nb-V microalloyed large forging steel

For large forging steel, it is impracticable to enhance cooling rate in case of thermal stress-cracking The cooling method followed normalizing is usually air cooling, hence the austenization temperature is the decisive normalizing parameter for microstructure and properties The study of this paper aims at investigating the effect of normalizing temperature on microstructure and properties of a Nb-V microalloyed large forging steel The evolutions of austenite, microstructure and precipitations of the tested steel were characterized The relations between the microstructure and properties were discussed The tested steel in this work with the diameter of Φ290 mm and the V content up to 0.095 wt% has never been investigated

2 Experimental material and procedures

The steel used in this work is a commercial HSLA steel The chemical composition is listed in Table 1 The round bar specimens with a length of 200mm and diameter 290 mm for normalizing is shown in Fig 1 They were cut from a Φ290 mm round forging In order to study the characteristic of microstructure at the corresponding normalizing temperature, cubic samples for quenching were wire-cut from the 1/2 radius of the Φ290 mm round forging The size of the quenching samples is 10 ×10 × 12 mm3 Both the normalizing and quenching process was conducted in a 45 kw box resistor-stove, the schedules of the process are given in Fig 2 The normalizing specimens were reheated at 820 ºC, 850 ºC, 880 ºC, 910 ºC and 940 ºC with soaking for 2 hours, respectively, and then were cooled by air with about a 0.03 ºC/s cooling rate In order to study the evolution of austenite, samples for interrupted heating by quenching were respectively reheated at 730 ºC, 760 ºC, 790 ºC, 820 ºC, 850 ºC, 880 ºC, 910 ºC and 940 ºC holding for 2 hours

After the normalizing process, blanks for metallographic observation and mechanical property test were wire-cut from the 1/2 radius of the normalizing samples along longitudinal axial direction as shown Fig 1 Metallographic observation direction for all test samples was parallel to the longitudinal section of the Φ290 mm round forging The microstructure of the samples was etched by a 4% nital solution The size and area fraction of ferrite and pearlite constituent were measured by software Image-Pro Plus For each specimen, at least 5 fields of view containing at least 400 grains were measured Thin foils for Transmission Electron Microscopy (TEM) were prepared using the twin-jet method and observed in a JEM-2100 transmission electron microscope

The average size and fraction of precipitate particles were statistically measured by averaging 5 fields of view containing at least 300 particles from the images of TEM In order to study the orientation characteristic of acicular pearlite, electron back scattered diffraction examinations were performed on a field emission gun scanning electron microscope

The blanks for tensile test were machined into standard tensile test specimens of 10 mm in gage diameter and 50 mm in gage length Tensile tests based on standard of ISO 6892-1: 2009 were carried out on a WDW-200D tensile testing machine at room temperature with a cross-head speed of 0.25 mm min-1 [9] The yield strength was determined by the 0.2% offset flow stress All results were repeated for three times and the average values were taken to describe the tensile properties of the test steel The Charpy V-notched specimens with cross section of 10 × 10 mm2, length of 55 mm, notch angle of 45° and notch depth of 2 mm were employed to study the -40 ºC impact fracture toughness on a ZBC2452-B impact testing machine according to ISO 148-1: 2006

Table 1 Chemical compositions of the tested steel (wt%)

Fig 1 The normalizing sample and sampling method

Fig 2 Normalizing and quenching process

3 Results and discussion

3.1 The evolution of austenite formation

The study of the austenite formation was carried out using a DIL805A high resolution dilatometer Cylindrical samples of 4 mm diameter and 10 mm length were used for the

experiments As shown in Fig 3, the relations between heating temperature(T) and expansion amount(ΔL) were analyzed to determine Ac1 and Ac3 at a constant heating rate of 10 ºC/min Since

some investigations have experimentally shown that a separation can be made between the pearlite to austenite and the ferrite to austenite transformation [10-12], an attempt was made to determine

the temperature (Acθ) at which this occurred The determination of Acθ can be only attempted if the

first contraction is perceived It is less evident to see from the dilatometric curve, but easier to

determine from the first derivative (dΔL/dT) as shown in Fig 3 In previous papers studied similar

steels, the authors showed that this first contraction was related to the dissolution of pearlite[13, 14] As shown in Fig 4(b) and 6(b), microstructure morphologies support the fact that the first contraction observed in the dilatometric curve is due to pearlite dissolution

Fig 3 Dilatometric curve and the first derivative

According to the results of dilatometer tests, the Ac1 and Ac3 of the tested steel were

determined as 725 ºC and 861 ºC, respectively Fig 4 shows microstructures obtained after interrupted heating by quenching at 730-940 ºC It can be seen that the specimen heated at 730 ºC mainly consists of white band and black band microstructure With increasing temperature from 730 ºC to 850 ºC, the black band microstructure gets wider and wider, the white band gets narrower and narrower accordingly There is no obvious banded microstructure in samples heated

Fig 4 Optical micrographs of quenched microstructure after heating at different temperatures

(a) 730 ºC, (b) 760 ºC, (c) 790 ºC, (d) 820 ºC, (e) 850 ºC, (f) 880 ºC, (g) 910 ºC, (h) 940 ºC Fig 5 shows the SEM micrographs of the quenched microstructure Fig 5(a)-Fig 5(e) corresponds to the micrographs of the white band microstructure in Fig 4(a)-Fig 4(e) Fig 5(f)-Fig 5(h) are typical micrographs corresponding to Fig 4(f)-Fig 4(h) It can be seen from Fig 5(a) that the microstructure mainly consists of ferrite as well as a small amount of martensite which are distributed at the ferrite boundaries Formation of martensite in ferrite band indicates that actual austenization has started at 730 ºC With increasing temperature to 760 ºC, net-like martensite can be seen clearly along the ferrite boundaries, which shows that ferrite boundaries is the preferential site for austenite nucleation and growth With further increasing of temperature from 760 ºC to 790 ºC, the net-like martensite grows into the ferrite Accordingly, the amount of ferrite is reduced remarkably Fig 5(d) and Fig 5(e) show that the microstructure is composed of martensite and a small amount of ferrite after quenching at 820 ºC and 850 ºC, respectively The acicular microstructure adjacent to the ferrite is turned out to be martensite as shown in Fig 5(i) The microstructure completely consists of martensite when the heating temperature is 880 ºC, 910 ºC, and 940 ºC, indicating that the actual austenization of ferrite band has been finished at about 880 ºC

The formation of acicular microstructures (finger-type austenite) during austenization was also observed by many researchers [15-18] It is quite possible that the formation of the fingers coincide with the position of former cementite plates that were perpendicular to the grain

Fig 5 SEM micrographs of quenched microstructure after heating at different temperatures

(a) 730 ºC, (b) 760 ºC, (c) 790 ºC, (d) 820 ºC, (e) 850 ºC, (f) 880 ºC, (g) 910 ºC, (h) 940 ºC, (i) micrographs of the acicular microstructure(TEM)

Fig 6 shows the SEM microstructures of the quenched microstructure Fig 6(a)-Fig 6(e) correspond to the micrographs of the black band microstructure in Fig 4(a)-Fig 4(e) Fig 6(f)-Fig 7(h) are typical micrographs corresponding to Fig 4(f)-Fig 4(h) It can be seen from Fig 6(a) that the microstructure consists of pearlite and ferrite, as well as a small amount of martensite which is distributed at the ferrite-pearlite boundaries With increasing temperature to 760 ºC, blocky martensite can be seen clearly around ferrite, and pearlite has also been dissolved simultaneously Fig 6(c) and Fig 6(d) show that the microstructure is composed of martensite and a small amount of ferrite after quenching at 790 ºC or 820 ºC The microstructure completely consists of martensite when the heating temperature is 850, 880, 910, or 940 ºC, indicating that the actual austenization of the pearlite bands has been finished at 850 ºC

Fig 6 SEM micrographs of quenched microstructures after heating at different temperatures

(a) 730 ºC, (b) 760 ºC, (c) 790 ºC, (d) 820 ºC, (e) 850 ºC, (f) 880 ºC, (g) 910 ºC, (h) 940 ºC Under the given conditions in this paper, the formation of austenite in initial ferrite and pearlite was asynchronous The nucleation and growth of austenite in initial pearlite was earlier and faster than that in ferrite The microstructure heredity of austenite on ferrite and pearlite will be shown below

3.2 Microstructure of normalized samples

Fig 7 shows the optical micrographs of the samples before and after normalizing As can be seen in Fig 7(a), as-forged microstructure consists of alternate bands of proeutectoid ferrite and pearlite EPMA was used to investigate the chemical characteristic across the banded microstructure in Fig 7(a) Figs 8(a) and 8(b) show manganese and silicon profiles, respectively Excellent correlation between microstructural banding and microchemical banding is apparent In other words, solute lean regions and solute rich regions are consistently associated with regions of proeutectoid ferrite and pearlite, respectively The results are in full agreement with the findings of

Thompson and Howell [19] They showed that microchemical banding of manganese was the cause of the microstructural banding observed in a similar steel

Fig 7 The optical micrographs for the specimens before and after normalizing

(a) as-forged, (b) 820 ºC, (c) 850 ºC, (d) 880 ºC, (e) 910 ºC, (f) 940 ºC

Fig 8 Concentration profiles from as-forged sample

(a) manganese profile, (b) silicon profile

In an attempt to determine the reliability of the data presented in Fig 8(a), the maximum and

minimum concentrations of manganese were estimated using the Scheil equation [20], which was developed for the limiting case of no diffusion in the solid, but complete mixing in the liquid

Cs = kC0(1- fs)k-1 (1)

where Cs is the concentration of solute in the solid at a given fraction solidified fs, C0 is the

concentration of solute in the alloy, and k is the equilibrium partition ratio, defined as

k = Cs/C1 (2)

where C1 is the concentration of solute in the liquid in equilibrium with the solid of concentration

Cs To simplify the analysis, k is assumed to be constant throughout the solidification range For manganese segregation k=0.71 and C0 is 1.45% Mn [21] Thus, Cs=1.06%Mn for fs=0.1, and

Cs=2.00%Mn for fs =0.9 These values are in reasonably good agreement with the maximum and minimum values of manganese concentration shown in Fig 8(a)

Fig 7(b) and Fig 9(a) show the inhomogeneous banded microstructure which consists of fine grain bands (FGB) and coarse grain bands (CGB) The FGB is composed of relatively large size of ferrite (initial ferrite) and acicular black microstructure The CGB consists of relatively fine ferrite (F) and pearlite The acicular microstructure as shown in Fig 9(b) is identified as lamellar pearlite by using SEM Hence, it is called acicular pearlite (AP) in this paper Color coded inverse

CGL

pole figure (IPF) maps of ferrite in CGB is given in Fig 9(c), which indicates that the adjacent acicular pearlite possesses the same crystal orientation

Fig 9 The microstructures obtained with the normalizing temperature 820 ºC

(a) OM , (b)SEM, (c) IPF map

From the above, complete austenization was finished at about 880 ºC, so there is coarse initial ferrite in the samples normalized at 820 ºC and 850 ºC When the temperature is 880 ºC, the microstructure is the finest With increasing temperature from 880 ºC to 940 ºC, abnormal growth of some pearlite nodules occurs, as shown in Fig 7(e) and (f) The average size of the pearlite nodules in the specimens normalized at 910 ºC and 940 ºC are about 38 and 44 μm in diameter, respectively These values are very similar to the largest austenite grain sizes in the specimens As shown in Fig 10(a), the largest austenite grain size is about 30 μm in diameter in specimen reaustenitized for 2 hours at 880 ºC The number density is very low Conversely, as shown in Fig 10(b) and (c), numerous large austenite grains with the diameter ranging from 40 to 60 μm were observed in the specimens reaustenitized for 2 hours at 910 ºC and 940 ºC

Fig 10 The micrographs for the specimens reaustenitized at different temperatures

(a) 880 ºC, (b) 910 ºC, (c) 940 ºC

The similarity in sizes and number densities for the large pearlite nodules and large austenite grains strongly suggests that the large pearlite nodules form within large austenite grains If it is assumed that air cooling (about 0.03 ºC/s) produces microstructure that are relatively close to equilibrium, and it is further assumed that the pearlite nodules form from a single austenite grain, then the size of the nodules can be estimated from the austenite grain size and the volume fraction of pearlite If, for simplicity, the austenite grains and the pearlite nodules are assumed to be spherical, then for a pearlite volume fraction of 0.39 and an austenite grain diameter of 60 μm, the diameter of the pearlite nodules would be about 44 μm Fig 7(f) and Fig 10(c) show that large pearlite nodules are about 30-50 μm in diameter, consistent with formation within the largest austenite grains Hence, it can be concluded that large, irregular pearlite nodules form in abnormally large austenite grains [22]

3.3 Precipitation

Fig 11 shows the bright field TEM micrographs and EDS analysis of the representative precipitations observed in the as-forged specimen The observed precipitations can be classified

into three different size ranges: 1-3 μm (cuboidal precipitations), 50-150 nm (spherical precipitations) and 5-15 nm (fine precipitations) The cuboidal precipitations are titanium-rich and niobium-contained (Ti, Nb)C (Fig 12a and d) The spherical precipitations are niobium-rich and titanium-contained (Nb, Ti)C (Fig 12b and e) and present a chain-like distribution The fine precipitations are vanadium-rich and niobium-contained (V, Nb)C (Fig 12c and f) The majority of the precipitations are duplex-type carbides

Statistics shows that there is no obvious difference of the precipitations in size and distribution between the specimens before and after normalizing Fig 12 shows the bright field TEM micrographs of the representative precipitations observed in the normalized specimens

Fig 11 Three typical TEM micrographs and EDS analysis of the precipitations

(a and d)cuboidal precipitation, (b and e)spherical precipitations, (c and f)fine precipitations

Fig 12 Three typical TEM micrographs of the precipitations in the specimens normalized at 850

ºC (a)cuboidal precipitation, (b)spherical precipitations, (c)fine precipitations

3.3 Mechanical properties

Performance requirements for the tested steel are listed here: Rp0.2≥265 MPa, Rm=450-600

MPa, A≥17%, KV2≥27 J(-40 ºC) Mechanical properties of the tested steels are ineligible before

normalizing as listed: Rp0.2=326 MPa, Rm=520 MPa, A=25%, KV2=15J The heat treated specimens exhibited superior mechanical properties over the original untreated specimen in terms of higher tensile strength, elongation and impact energy as summarized in Table 2