Effects of Nb Addition on the Microstructure and Mechanical Properties in Friction Welded Joints of Non-Quenched and Tempered Steels

Bongjun Kim; Joonoh Moon; Jun-Yun Kang; Jungwon Lee; Yonghyun Kim; Jinseon Kim

doi:10.5781/JWJ.2025.43.6.12

Abstract

In this study, the effects of Nb addition on the microstructure and mechanical properties of friction welded joints of non-quenched and tempered (NQT) steels were investigated. For this purpose, NQT steels with different Nb content were bonded with high carbon S45C steel by friction welding. The microstructures of welded joints were characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM). For evaluating the mechanical properties of welded joints, the tensile, Charpy U-notch impact, and Vickers hardness testing were performed. There is no fusion zone in the friction welded-joints, and thermomechanically affected zone (TMAZ) and heat-affected zone (HAZ) are observed as the distance from the bond line between NQT steels and S45C steel increases. The microstructures in TMAZ and HAZ of S45C steel consist of grain boundary ferrite (GBF) and pearlite. NQT steels consist of bainite and GBF in TMAZ and HAZ, and the effective grain sizes in TMAZ is larger than those in HAZ. Nb addition contributed to fine precipitation of MC carbide in NQT steel, making TMAZ and HAZ structure fine and finally enhancing mechanical properties in friction welded-joint.

Key words: Non-quenched and tempered steel, high carbon S45C steel, Friction welding, Bond line, Heat-Affected Zone (HAZ), Thermomechanically affected zone (TMAZ)

1. Introduction

The demand for hydraulic cylinders has continued to increase due to the recent trend toward automation in the construction and manufacturing industries and the growing need for large construction equipment. Hydraulic cylinders convert hydraulic energy into mechanical energy and must support high loads and withstand repeated fatigue loading. The durability of both the base material and the welded joints is therefore a critical factor. High-carbon steels are currently the most widely used materials for hydraulic cylinders¹⁾. Through quenching and tempering (QT) heat treatment, high-carbon steels can achieve tempered martensite microstructures that provide excellent strength and ductility, making them suitable for hydraulic cylinder applications. In recent years, efforts have intensified to reduce manufacturing costs and carbon emissions by omitting the QT heat treatment step traditionally employed in hydraulic cylinder production while still achieving mechanical properties comparable to QT-treated steels. This has led to active development of non-quenched and tempered steels (NQT steels) that exhibit equivalent mechanical performance without heat treatment¹^-⁶⁾.

Welding and joining processes are essential in the manufacture of hydraulic cylinders, particularly for joining high-strength materials such as high-carbon steels and NQT steels¹⁾. Various welding techniques have been applied, and friction welding is one of them¹^,⁵⁾. In friction welding, two materials are brought into contact and rotated rapidly relative to each other to generate frictional heat. The localized melting occurs by this heat and the liquid metal is then expelled during the subsequent upsetting stage, during which pressure is applied to join the materials⁶^-⁹⁾. Because the molten metal is expelled, a conventional fusion zone does not form. Instead, a solid-state bond line develops at the interface, with heat-affected zones (HAZs) forming on both sides of the bond line⁵⁾. During the upsetting stage, dynamic recrystallization occurs in regions near the bond line due to the applied force, producing a microstructure distinct from that of the HAZ farther from the interface. This region is classified as the thermomechanically affected zone (TMAZ).

In this study, the relationship between microstructure and mechanical properties in friction-welded joints of NQT steels developed for hydraulic cylinder applications was investigated, with particular emphasis on the effect of Nb addition. Two NQT steels were prepared, one without Nb and one containing 0.01 percent Nb. Each was friction welded to S45C high-carbon steel. The friction welding characteristics of the Nb-free NQT steel have been reported previously⁶⁾, and in the present study additional analyses were conducted to enable comparison with the Nb-containing NQT steel. The microstructure and precipitate distribution in the friction-welded joints were examined using optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). Mechanical properties were evaluated through Vickers hardness testing, room-temperature tensile testing, and U-notch impact testing. Finally, the relationship between microstructure evolution and mechanical properties in the friction-welded joints and the effects of Nb addition are discussed.

2. Experimental Methods

The chemical compositions of the two NQT steels and the S45C high-carbon steel used in this study are summarized in Tables 1 and 2, respectively. As shown in Table 1, the NQT steels are denoted as Nb-free steel and Nb-0.01 steel depending on whether Nb was added. According to previous research, the addition of Nb to NQT steels has been reported to improve mechanical properties due to grain refinement and the formation of fine precipitates¹⁾. The two NQT steel alloys used in this study were designed to investigate these Nb-induced effects. For the NQT steels listed in Table 1, equilibrium phase diagrams were calculated using the commercial thermodynamic software Thermo-Calc (TCFE 12 database) and used to support microstructural analysis. Table 3 presents the welding conditions employed during friction welding between the NQT steels and the S45C high-carbon steel.

Table 1

Chemical composition of NQT steel investigated in this study (wt.%)

	C	Si	Mn	V	Nb	Fe
Nb-free	0.28	0.97	1.61	0.14	-	Bal.
Nb-0.01	0.26	0.94	1.45	0.15	0.01	Bal.

Table 2

Chemical composition of S45C steel (wt.%)

C	Si	Mn	P	S	Fe
0.42~0.48	0.15~0.35	0.6~0.9	< 0.03	< 0.035	Bal.

Table 3

Friction welding conditions performed in this study

Initial friction process			Friction process		Upsetting process
Force, kgf	RPM	Time, s	Force, kgf	RPM	Force, kgf	Time, s
13,100	400~500	20~25	46,288	42~500	62,882	15~20

The microstructures of the base materials and the different regions of the friction-welded joints (bond line, TMAZ, and HAZ) were examined using OM, SEM, and EBSD. For OM and SEM observations, specimens were ground using SiC papers, polished, and etched with 3 percent Nital solution. EBSD analysis was performed after polishing with colloidal silica suspension. TEM samples were prepared using twin-jet electropolishing.

Mechanical properties of the base materials and the friction-welded joints were evaluated through Vickers hardness testing, room-temperature tensile testing, and low-temperature impact testing. Vickers hardness profiles were measured across the joint from the NQT steel base material, through the bond line, and into the S45C base material under a load of 200 g. Tensile specimens were extracted in the cross-weld direction with the bond line at the center, following the ASTM E8/E8M standard. Charpy U-notch impact tests were conducted from room temperature down to −40 °C in accordance with ASTM E23.

3. Experimental Results and Discussion

3.1 Microstructure and Mechanical Properties of the Base Materials

Fig. 1 presents the base-metal microstructures of the two NQT steels used in this study. Fig. 1(a) and 1(b) and Fig. 1(c) and 1(d) show the OM and SEM microstructures of the Nb-free steel and the Nb-0.01 steel, respectively. As shown in the images, both NQT steels exhibit the typical ferrite and pearlite microstructure. Fig. 2(a) and 2(b) show the EBSD inverse pole figure (IPF) maps of the Nb-free steel and the Nb-0.01 steel. It is evident from Fig. 2 that the addition of Nb (Fig. 2(b)) results in grain refinement compared to the Nb-free steel. The grain size of the Nb-0.01 steel is smaller than that of the Nb-free steel, which is attributed to the increased fraction of fine precipitates formed by Nb addition and the consequent suppression of grain growth. Fig. 3 presents the TEM analysis results for the NQT steels. As shown in Fig. 3(a) and 3(c), both steels contain large quantities of nanoscale precipitates with average sizes below approximately 50 nm. Based on the EDS analyses in Fig. 3(b) and 3(c), VC precipitates were identified in the Nb-free steel, whereas (V,Nb)C precipitates were observed in the Nb-0.01 steel. These precipitation behaviors and the differences introduced by Nb addition can be explained using the thermodynamic calculations presented in Fig. 4. As shown in Fig. 4(a) and 4(b) show the equilibrium phase diagrams of the Nb-free steel and the Nb-0.01 steel calculated using Thermo-Calc software. As shown in Fig. 4(a), in the Nb-free steel, VC begins to precipitate at approximately 900 °C due to the presence of V, and its volume fraction increases as the temperature decreases. In contrast, in the Nb-0.01 steel (Fig. 4(b)), the combined addition of V and Nb results in (V,Nb)C precipitation beginning at approximately 1100 °C, and the calculated precipitate fraction is higher than that of the Nb-free steel. These results indicate that Nb addition enhances the high-temperature stability of MC-type carbides and increases the precipitate fraction. Consequently, the pinning effect on grain boundaries increases, leading to stronger suppression of grain growth. This explains why the Nb-0.01 steel exhibits a finer grain size than the Nb-free steel, as shown previously in Fig. 2.

Fig. 1

Base metal microstructures of NQT steels, (a-b) OM and SEM micrographs of Nb-free steel and (c-d) OM and SEM micrographs of Nb-0.01 steel. PF: Polygonal ferrite, P: Pearlite

Fig. 2

EBSE IPF maps and effective grain size of the base metals, (a) Nb-free steel and (b) Nb-0.01 steel

Fig. 3

TEM analyses of the base metals, (a-b) STEM image and EDS analyses for fine precipitates in Nb-free steel and (c-d)) STEM image and EDS analyses for fine precipitates in Nb-0.01 steel

Fig. 4

Equilibrium phase fraction diagrams of NQT steels calculated by Thermo-Calc., (a) Nb-free steel and (b) Nb-0.01 steel

Fig. 5 presents the mechanical properties of the two NQT steels. As shown in Fig. 5(a), the Nb-0.01 steel exhibits nearly twice the impact absorption energy of the Nb-free steel at all test temperatures. This improvement is attributed to the refined grain structure produced by Nb addition, which suppresses crack propagation during impact loading. Fig. 5(b) compares the tensile properties of the two base metals. All tensile properties, including tensile strength, yield strength, and elongation, increase with Nb addition. These improvements are attributed to both grain refinement and enhanced precipitation strengthening introduced by Nb.

Fig. 5

Mechanical properties in the base metals of NQT steels, (a) Charpy U-notch impact toughness and (b) tensile properties at room temperature

3.2 Microstructural Analysis of the Friction-Welded Joints

As shown in Fig. 6(a) and 6(b) show the SEM microstructures at selected locations within the friction-welded joints of the Nb-free steel and S45C high-carbon steel, and of the Nb-0.01 steel and S45C high-carbon steel, respectively. Regardless of whether Nb was added to the NQT steel, the friction-welded joints exhibit similar microstructural features. In the TMAZ and HAZ of the S45C steel, grain boundary ferrite (GBF) and pearlite are observed, and the grain size is larger in the TMAZ than in the HAZ. In the NQT steel, bainite is formed in the TMAZ, whereas both bainite and GBF appear in the HAZ. Similar to the S45C steel, the grain size in the NQT steel is coarser in the TMAZ than in the HAZ. Fig. 7 presents EBSD microstructural maps for various locations within the friction-welded joints. In both the TMAZ and HAZ of the NQT steels, lath structures develop due to bainite formation. The microstructures are similar regardless of Nb addition. EBSD analysis was used to determine the prior austenite grain size (PAGS) in the TMAZ and HAZ of the NQT steel joints, and the results are summarized in Fig. 8. As shown in the figure, both the TMAZ and HAZ of the Nb-0.01 steel exhibit finer grains compared with the Nb-free steel. This grain refinement is attributed to suppression of grain growth during the thermal cycles of friction welding, which results from the increased precipitate fraction induced by Nb addition.

Fig. 6

SEM micrographs in the friction welded joints between, (a) Nb-free steel + S45C steel and (b) Nb-0.01 steel + S45 steel. GBF: Grain boundary ferrite (GBF), P: Pearlite, B: Bainite

Fig. 7

EBSD IPF maps in the friction welded joints between, (a) Nb-free steel + S45C steel and (b) Nb-0.01 steel + S45C steel

Fig. 8

Measured prior austenite grain size in the friction welded joints of NQT steels

3.3 Mechanical Properties of the Friction-Welded Joints

As shown in Fig. 9 shows the Vickers hardness distribution measured across the friction-welded joints. Regardless of Nb addition, the TMAZ and HAZ of the S45C steel exhibit similar hardness values. In contrast, in the NQT steel, hardness increases in both the TMAZ and HAZ with Nb addition. This increase corresponds to enhanced precipitation strengthening and grain refinement, as also supported by the PAGS data in Fig. 8. Although both the S45C steel and the NQT steel exhibit larger PAGS in the TMAZ than in the HAZ, the TMAZ shows higher hardness. This behavior is attributed to the higher fraction of soft ferrite in the HAZ, as observed in Fig. 6.

Fig. 9

Hardness profile across the friction welded joints

Fig. 10 and 11 present the impact and tensile properties measured using specimens machined in the cross-weld direction with the bond line at the center. For the impact tests, the notch was machined along the bond line. As shown in Fig. 10, similar to the base-metal results, the friction-welded joint of the Nb-0.01 steel exhibits superior low-temperature impact toughness compared with the Nb-free steel. This improvement reflects the grain refinement of the NQT steel weld region. In contrast, in the Nb-0.01 steel (Fig. 5(a). This reduction results from grain coarsening in the TMAZ and HAZ during the thermal cycle of friction welding and from the effects of dissimilar joining with the S45C steel, as observed in Fig. 2 and Fig. 8. Fig. 11(a) presents the tensile properties of the friction-welded joints. The Nb-free and Nb-0.01 steel joints show similar tensile behavior. This similarity occurs because, as shown in Fig. 9, the HAZ adjacent to the base metal has the lowest hardness, and fracture consistently occurs in this region during cross-weld tensile testing, regardless of Nb addition. As shown in Fig. 11(b) shows photographs of the tensile specimens after testing, confirming that fracture occurred in the HAZ near the base metal rather than at the bond line. The images also show that tensile deformation is concentrated near the fracture region in either the NQT steel or the S45C steel. Therefore, the elongation values shown in Fig. 11(a) do not fully represent the intrinsic tensile ductility of the friction-welded joints due to this localized deformation behavior.

Fig. 10

Charpy U-notch impact toughness of the friction-welded joints

Fig. 11

Mechanical properties of the friction welded joints, (a) tensile properties at room temperature and (b) photos of the specimen after tensile tests

4. Conclusions

Two NQT steels with different Nb contents were fabricated and friction welded to S45C high-carbon steel. The microstructures of the base metals and the various regions of the friction-welded joints were analyzed, and the mechanical properties were evaluated. The major findings of this study are summarized as follows.

1) The base metals of the NQT steels consisted of polygonal ferrite and pearlite. Finer grain sizes were observed with the addition of Nb. This refinement is attributed to the increased fraction of nanosized MC carbides and the consequent suppression of grain growth.

2) Both the room-temperature tensile properties and low-temperature impact toughness of the NQT steels improved with Nb addition. These improvements result from increased precipitation strengthening and enhanced grain growth suppression associated with the higher fraction of fine precipitates.

3) The friction-welded joints exhibited similar microstructures regardless of Nb addition. In the TMAZ and HAZ of the S45C steel, GBF and pearlite were observed, and the TMAZ exhibited larger grain sizes than the HAZ. In the NQT steel, bainite formed in the TMAZ, whereas both bainite and GBF were observed in the HAZ. Similar to the high-carbon steel, the TMAZ of the NQT steel showed relatively coarser grains than the HAZ. However, both the TMAZ and HAZ of the Nb-0.01 steel exhibited finer grain sizes than those of the Nb-free steel.

4) Hardness measurements across the friction-welded joints indicated that the hardness of the TMAZ and HAZ in the NQT steels increased with Nb addition. This increase is attributed to enhanced precipitation strengthening and reduced grain size in both regions.

5) The friction-welded joint produced using the Nb-0.01 steel demonstrated superior impact toughness compared with that produced using the Nb-free steel. This improvement corresponds to the grain refinement effect induced by Nb addition.