Effects of Chemical Composition on Welding HAZ Softening of High-Strength Pipeline Steels

Abstract

With the increase in strength of pipeline steels manufactured by thermomechanical control process (TMCP), the softening of the welding heat-affected zone (HAZ) becomes another important factor affecting the properties of welded steel pipes and the safety of pipeline operation. In this work, based on the actual welding process of steel pipes, the strength, phase transformation, and microstructure of the HAZ of six pipeline steels with different chemical compositions were studied by using a thermomechanical simulator, and the effect of chemical composition on the softening of HAZ was discussed. Results show that the strength of HAZs is significantly influenced by the peak temperature, and the softening zone mainly occurs in fine-grained HAZ (FGHAZ) when peak temperature is 900~1000 °C. Meanwhile, the degree of softening is also affected by the chemical composition of the steels. The effects of peak temperature and chemical composition of the steels on the strength of the HAZs when the peak temperature is over Ac₃ are attributed to their effect on the austenite transformation during the heating process, and then the effect on phase transformation during the cooling process and final microstructure. The strength of the HAZs is linearly related to the beginning phase temperature during the cooling process, and the strength of sub-HAZs at the same peak temperature is linearly related to the value of carbon equivalent (Ceq) of steels. Therefore, controlling the appropriate value of Ceq is necessary to improve the softening of HAZs for high-strength pipeline steels.

1. Introduction

Oil and natural gas are the leading sources of energy across the world. Pipeline transportation is the earliest developed efficient method of energy transmission [1]. As early as 1926, the American Petroleum Institute (API) issued a line pipe specification, which only included three steel grades: 25A, A, and B [2]. Ever-increasing demand for oil and natural gas requires the construction of a gas transmission pipeline with large transportation capacity; this, in turn, necessitates a large-diameter, thick-walled, high-strength pipeline steel pipe [3,4,5]. Pipeline steel grades have rapidly advanced from grade B, X42~X65 to X70, 80, and even X90~X120, which were listed in the new edition of API Specification 5L [6]. Yet up to now, the high-strength X80 pipeline steel has been widely applied [7,8,9], the X90, X100 pipeline steels have also been extensively studied, and several test pipelines have been built [10,11]. The high-strength pipeline steels are usually produced by advanced low-carbon microalloyed steels and the TMCP process; thus, the refined microstructure and the balance of high strength and excellent toughness have been obtained [12,13,14,15,16,17]. However, the pipeline steel pipes are usually produced by submerged arc welding; as a consequence, the refined microstructure of the pipeline steels manufactured by the TMCP process will be destroyed in the welding heat-affected zone (HAZ) that undergoes high temperatures during the welding thermal cycle [1,2], so the balance of strength and toughness will be deteriorated [1,2]. Therefore, the HAZ is considered the weakest area of mechanical properties, which significantly affects the safety of pipeline operation. The weldability of pipeline steels is always the focus of research in the field of pipeline steel [2,18,19,20,21].

Generally, the research on the mechanical properties of welding HAZ for the pipeline steels mainly focuses on the toughness of the welding coarse grain HAZ (CGHAZ), because the unfavorable microstructures of coarse austenite, martensite, and/or bainite resulted from high peak temperature and cooling rates. The unfavorable microstructures of CGHAZ significantly deteriorate the mechanical properties, especially toughness [22,23]. However, the deterioration of mechanical properties in HAZs depends on the chemical composition of steels and welding process parameters [24,25]. With the increase in strength and thickness of steel pipe, the increase in necessary welding heat input energy results not only to the deterioration of toughness, but also to the deterioration of strength [26,27,28]. In consequence, the softening phenomenon of the welding HAZ of thick-wall high-strength steel pipes becomes prominent.

It is different from the toughness deterioration of the welding HAZ, which mainly occurs in the CGHAZ, the softening of the welding zone occurs in the fine-grained HAZ (FGHAZ) [26,27,28] and intercritical HAZ (ICHAZ) [29]. The reason for the softening of the welding HAZ is related to the low peak temperature and the short residence time, which is only slightly lower or higher than the Ac₃, resulting in the formation of austenite grains with small and poor uniformity. Finally, the high-temperature transformation microstructure, such as PF and QF, is obtained. In this case, the chemical composition of the pipeline steels and the corresponding welding parameters are the most critical factors for the softening effect of welding HAZ [27]. Our previous research results show that a little change in Cr, Mo, Nb, and other alloying elements can significantly affect the softening of welding HAZ [28]. Therefore, a proper design of chemical composition is of great significance for improving the weldability of pipeline steel. However, for the high-strength pipeline steels, the chemical composition is complex with alloying elements of Mn, Cr, Ni, Mo, Cu, and microalloying elements of Ti, Nb, and so on; thus, it is difficult to accurately determine the influence of alloying elements on the softening of the welding heat-affected zone [30]. Hamada et al. [27] pointed out that the softening of HAZ may be related to the welding sensitivity coefficient (Pcm) and carbon equivalent (Ceq) for an X80 UOE pipe. Our previous results of statistical analysis of X80 LSAW pipes show that the strength of base metals and weld joints is linear to the Ceq and Pcm, and the slope of weld joints is higher than that of base metals with the increase in Ceq and Pcm. The results indicate that the increase in the Ceq and Pcm may improve the softening of HAZ [31]. Therefore, to control the softening of welding HAZ, it is necessary to further confirm the mechanism of alloy elements and Ceq.

In this work, the strength of HAZs with different peak temperatures of six commercial pipeline steels was studied by using a physical simulation method. The effects of peak temperature on microstructure and strength of HAZs of the six pipeline steels were investigated; meanwhile, the phase transformation temperature during welding thermal cycle cooling processes was measured. Furthermore, the effects of chemical composition on microstructure, strength, and phase transformation temperature were analyzed, and from the point of view of the Ceq, the relationship of the strength with the chemical composition of the steels at different temperatures was discussed. These research results provide a useful reference for the chemical composition design of high-strength pipeline steels for weakening the softening of HAZs.

2. Materials and Methods

Test pieces of six steels were cut from the commercial API X80 (steel D, E and F) and X90 (steel A, B and C) high-strength pipeline steel plates of thickness of 22 mm produced by different steel mills, and the chemical composition of the steels are shown in

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

Steels	Grade	C	Si	Mn	P	S	Cr	Mo	Ni	Cu	Nb	Ti	N	Fe	Ceq
A	X90	0.06	0.24	1.80	0.009	0.002	0.21	0.28	0.234	0.213	0.095	0.014	0.004	Bal.	0.488
B	X90	0.05	0.24	1.95	0.008	0.002	0.31	0.30	0.425	0.021	0.062	0.014	0.004	Bal.	0.527
C	X90	0.06	0.18	1.84	0.008	0.002	0.30	0.19	0.384	0.203	0.080	0.011	0.004	Bal.	0.504
D	X80	0.06	0.22	1.65	0.010	0.003	0.23	0.01	0.215	0.019	0.079	0.016	0.006	Bal.	0.399
E	X80	0.06	0.22	1.64	0.007	0.003	0.22	0.01	0.224	0.139	0.079	0.017	0.006	Bal.	0.404
F	X80	0.06	0.20	1.79	0.011	0.001	0.26	0.24	0.037	0.016	0.062	0.014	0.004	Bal.	0.462

Note: Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15.

, meanwhile, the values of carbon equivalent (Ceq) of steels recommended by the International Institute of Welding (IIW) are also listed in Table 1 [6]. The test pieces were machined into specimens of ϕ 10 × 80 mm (Figure 1a) for welding HAZ simulation, and the welding thermal cycle simulation experiments were carried out on a Gleeble-3500 thermal–mechanical simulator (DSI, New York, NY, USA).

In order to study the effects of peak temperature on microstructure and properties of HAZ of the heavy thickness and high-strength pipeline steel pipe, the heat inputs of 4.0 kJ/mm were selected to simulate the tandem four-wire serial submerged arc welding process parameters of the steel pipe manufacturing process. The peak temperatures from 1350 °C to 650 °C were selected to simulate the welding HAZ from CGHAZ to the sub-intercritically reheated zone. The thermal cycle parameters were determined by ANSYS 16.0 analysis and simulated by thermal–mechanical simulator (Figure 1c), the main cooling parameter of the time of cooling from 800 °C to 500 °C (t_8/5) is about 44 s. Meanwhile, the phase transformation temperature during cooling process was measured by dilation method. After simulation, the specimens were further machined into the tensile samples specified in GB/T 228.1-2021 [32] (Figure 1b), and the tensile test was performed on an INSTRON 5585H electric tensile testing machine (INSTRON, Norwood, MA, USA). Three effective samples were made from each group of test schemes, and the results of tensile properties were averaged to ensure the accuracy of the test results. Some types of specimens after thermal cycle were selected for microstructure observation. The microstructure observations were conducted using optical microscopy and scanning electron microscopy (SEM) with electron back-scattered diffraction (EBSD) technique.

3. Results

3.1. As-Rolled Microstructure and Tensile Properties of the Pipeline Steels

Figure 2 and

Table 2. The tensile properties of test steels.

Steels	A	B	C	D	E	F
Yield strength/MPa	669	702	663	574	543	665
Tensile strength/MPa	753	785	773	688	648	769
Yield ratio	0.89	0.89	0.86	0.83	0.84	0.86

show the as-rolled microstructure and tensile properties of the six pipeline steel plates. The microstructure of the low-carbon microalloyed steels are nonclassical and more complex, which may consist of polygonal ferrite (PF), quasi-polygonal ferrite (QF or massive ferrite (MF)), granular bainite (GB), bainite (BF). Nevertheless, some differences in the microstructure of the six steels can be found (Figure 2), which causes the change in tensile strength (Table 2). As shown in Figure 2a,b, the microstructure of Steel A and Steel B mainly consists of BF and QF(or MF), as well as a few PF that are mainly distributed along the elongated prior austenite grain boundary. The microstructure of Steel A and Steel B, dominated by BF, provides high strength (Table 2). Meanwhile, the microstructure of Steel C~Steel F changes to the mixture dominated by QF and a few PF and GB, and the prior austenite grain boundaries disappear, and the microstructure becomes finer (Figure 2c–f). The microstructure changes from being dominated by BF to QF will result in a decrease in the strength of steels (Table 2).

The microstructure and mechanical properties of steels are mainly attributed to their chemical composition and TMCP parameters. Although the steels produced by different steel mills may have adopted different TMCP processes, it is worth noting that the strength of steels is highly correlated with its chemical composition. Combining Table 1 with Table 2, with the increase in the amount of C and alloy elements, especially the value of Ceq, the strength of steels increases. However, some differences can also be observed. For instance, the Ceq values of steel A and steel F are lower than that of Steel C, but their strength is comparable to that of steel C; In addition, Both of Steel D and E have similar alloy contents, but the strength of Steel D is significantly higher than that of Steel E. the results are attributed to the effect of the TMCP processes.

During the welding thermal cycle, the welding HAZ will undergo a rapid heating and cooling process. The microstructure and mechanical properties of the welding HAZ depend on the phase transformation during the welding heat cycle. While the phase transformation of welding HAZ may be significantly influenced by the chemical composition and microstructure of the steel, it then influences the final microstructure and mechanical properties of the HAZ.

3.2. Strength of HAZ of High-Nb Pipeline Steels

Figure 3 shows the effect of the peak temperature on the strength of HAZ of the six high-Nb pipeline steels. The strength of the HAZ of the six pipeline steels shows almost the same trend with the change in peak temperature. As shown in Figure 3a,b, for the high-strength pipeline steels, the HAZ of the pipeline steels shows higher strength when the peak temperature is 1350 °C, and then the strength decreases with the decrease in peak temperature from 1350 °C to 1000~900 °C. Meanwhile, the strength inversely increases to a higher value when the peak temperature further decreases to 730 °C, and then the strength decreases with the decrease in the peak temperature to 650 °C. The results indicate that the softening zone of strength occurs in a range of peak temperatures from 1000 °C to 900 °C.

Combining Table 1 and Table 2, the softening of HAZ seems to be related to the chemical composition and strength of the steels. Comparing the strength of the HAZ (Figure 3) with the steel plate (Table 2), the maximal softening rate was calculated based on the lowest strength of HAZ and the strength of steel, and the results is shown in Figure 3c. With the decrease in the amount of alloying elements of the steel, the strength of HAZ decreases, i.e., the softening rate increases. For example, the softening rate of Steel B with a higher value of Ceq of 5.24 is only about 6.0%, while the softening rate of Steel E with a lower value of Ceq of 4.04 increases to 15.3%. Besides, comparing Figure 3a,b, the effects of peak temperature on the yield strength is higher than the tensile strength; as a result, the softening rate of tensile strength is lower than that of yield strength (Figure 3c). The results indicate that a softening zone occurred in HAZ, which strongly relates to the chemical composition of the pipeline steels. Therefore, it is necessary to investigate the influence of alloying elements on tensile properties during the welding thermal cycle, particularly, yield strength, to weaken the softening effect of the welding thermal cycle.

3.3. Effect of Peak Temperature on Microstructure of HAZ

During the welding thermal cycle, the transformed microstructure in HAZ strongly depends on the peak temperature, which then affects the mechanical properties. From Figure 3, for all six steels, the change in strength with the peak temperature mainly occurs at 900 °C and 730 °C, which corresponds to the fine grain HAZ (FGHAZ) and intercritical HAZ (ICHAZ). Besides, the coarse-grain HAZ (CGHAZ) is considered to be one of the most severe toughness degradations in the HAZ. Therefore, the microstructure in HAZ with peak temperatures of 1350 °C, 900 °C, and 730 °C, which includes the typical sub-HAZ of CGHAZ, FGHAZ, and ICHAZ, was selected to be analyzed.

Figure 4 shows the metallographic microstructure of the CGHAZ of the six steels after thermal simulation with a peak temperature of 1350 °C. The microstructure of all steels mainly consists of BF and little GF, in which the prior-austenite grain boundary is clearly visible, and plenty of film-like, rod-like martensite/austenite (M/A) constituents and little dot-like M/A constituents are distributed in the matrix. The prior-austenite grain size is similar for all steels because the steels have a high content of Nb, and the NbC precipitates strongly inhibit the growth of the original austenite grain [31]. However, some differences in the morphology of bainite for the steels with different chemical compositions can still be found. The size of block and lath of BF seems to be a decreasing trend with the decrease in content of C and alloy elements of the steels, i.e., Ceq. Meanwhile, the size of M/A constituents decreases, and the number of M/A constituents increases. The result can be confirmed by the EBSD analysis result, as shown as in Figure 5. Figure 5 shows the IQ map with a high-angle grain boundary. The effective grain size, such as block and lath of BF of Steel E and D, is larger than that of other steels, and the density of high-angle grain boundaries is lower. With the increase in Ceq of the steels, the effective grain size of BF decreases, and the density of high-angle grain boundaries increases.

Figure 6 and Figure 7 show the metallographic microstructure and the IQ map with high-angle grain boundary of FGHAZ of the six steels after thermal simulation with a peak temperature of 900 °C. Comparing the microstructure of CGHAZs as shown in Figure 4, the microstructure changes significantly, which results to the microstructure being dominated by fine QF and PF, and some fine dot-like M/A constituents distribute in the matrix or between QF and/or PF grains. Meanwhile, the effect of the chemical composition of the steels on microstructure seems to show a similar role as that in the CGHAZs. With the increase in Ceq of steels, the amount of PF decreases, and the amount of QF increases; meanwhile, the size decreases, and the density of high-angle grain boundaries increases (Figure 7).

In addition, at the lower peak temperature that is slightly higher than Ac₃, the microstructure of the FGHAZs may be affected by the initial microstructure of the steel plate. The trace of as-rolled banded structure can be observed, especially for the steels with lower Ceq values (Figure 6d,e). The banded structure mainly consists of the QF and/or PF bands and the BF bands. The results indicate that the formed austenite is non-uniform, thus affecting the microstructure during the welding thermal cycle. It is a major reason that a large amount of PF and QF in the microstructure causes the strength to decrease.

Figure 8 shows the optical microstructure of ICHAZ of the six steels after thermal simulation with a peak temperature of 730 °C. Comparing the as-rolled microstructure of steels (Figure 2), the microstructure has a little change, and still retains the as-rolled characteristic. The microstructure of BF and QF appears to have degenerate characteristics; as a result, the microstructure shows a coarsening trend, and some larger-size ferrite grains occur in the microstructure. The change in microstructure after the thermal welding cycle only appears as a tempering microstructure feature. Therefore, the results indicate that the sub-HAZ after simulation with a peak temperature of 730 °C is still in the aging HAZ (AHAZ).

3.4. Effect of Peak Temperature on Phase Transformation Temperature

The results, as stated above, show that the peak temperature significantly affects the microstructure and strength of the HAZ, and the weakening zone occurs in the HAZ. Meanwhile, the changes in microstructure and strength strongly depend on the chemical composition of the steels, which is attributed to the effect of the chemical composition on phase transformation during the welding thermal cycle. Thus, the effects of peak temperature on phase transformation were investigated. Figure 9 shows the effects of peak temperature and chemical composition of the six steels on the temperature of the beginning phase transformation during the cooling process. As shown in Figure 9a, the beginning phase transformation temperature of all six steels during the cooling process decreases with the increase in peak temperature, which corresponds to the microstructure that changes from PF and QF transformed at higher temperature to BF and GB transformed at lower temperature. In addition, from Figure 9a, it can be seen that the chemical composition of the steels also has a significant effect on the beginning phase transformation during the cooling process. The beginning phase transformation temperature decreases with the increase in the content of C and alloyed elements.

However, the steels are typical low-carbon microalloying steels with complex alloying elements, such as Cr, Ni, Mo, Cu, and so on; therefore, the carbon equivalent of Ceq is chosen as a careful consideration of the influence of alloying elements on the beginning phase transformation temperature. Figure 9b shows the effect of the Ceq on the beginning phase transformation temperature during the cooling process. It can be seen that the changes in the beginning phase transformation temperature with the Ceq show a linear trend at every peak temperature, i.e., the beginning phase transformation temperature decreases with the increase in the Ceq. The results suggest that the chemical composition of high-strength pipeline steels significantly affects the phase transformation during the welding thermal cycle, thus affecting the microstructure and properties of the HAZs. Therefore, from the point of view of controlling the HAZ softening, the controlling chemical composition of high-strength pipeline steels is also very important.

4. Discussion

The welded joints, especially the welding HAZs, are considered as the weakest area of the weld pipes, because of the degradation of mechanical properties in the welding HAZs caused by the disruption of the refined rolling organization during the weld thermal cycle [2,10]. The variation in the microstructure and mechanical properties of the welding HAZ is attributed to the phase transformation during the heating and cooling process of the welding thermal cycle [22], while the phase transformation is affected by the chemical composition of the steels [24]. Therefore, the chemical composition, which determines the weldability, has always been an important parameter of pipeline steels [27]. The weldability of pipeline steels is usually evaluated by the Ceq and Pcm, and the maximum value of Ceq and Pcm is specified in specifications such as API 5L [6]. However, the Ceq usually evaluates the toughness of HAZs, especially CGHAZs, and the embrittlement worsens with the increase in the Ceq value, which may result in an increase in the tendency of detrimental microstructures, such as coarsened austenite grains, martensitic, and bainitic microstructures.

For the high-strength low carbon microalloying pipeline steels, the CGHAZ still remains good toughness [28,33], because the growth of prior austenite grain in CGHAZ can be effectively retard by the dispersed microalloy carbonitride particles [34,35]. Meanwhile, another problem emerges: the softening of the welding HAZ becomes a key problem. However, the softening zone occurs in the FGHAZ rather than the CGHAZ, as shown in Figure 3. The strength of HAZ decreases with the increase in peak temperature from 730 °C to 900 °C, and then increases with the further decrease in peak temperature from 900 °C to 1350 °C. The change in strength of HAZs is mainly attributed to the fact that the austenite transformation is affected by the peak temperature during the heat process, and then affects the phase transformation and room temperature microstructures during the cooling process.

The austenite transformation temperatures (Ac₁ and Ac₃) calculated by the chemical composition of all steels are in the range of 702~708 °C and 818~826 °C, respectively. Nevertheless, the Ac₁ and Ac₃ significantly increase due to the high heating rate during the welding thermal cycle and increase to a range of 760~780 °C and 915~940 °C, respectively. In this case, as the peak temperature is 730 °C, although the peak temperature is over Ac₁, the HAZs are still in the aging sub-intercritical zone, and the microstructure does not greatly change, and a tempered microstructure is obtained (Figure 8). While the strength of the HAZs may increase rather than decrease (Figure 3), the results are similar to those in reference [24,28,29,35,36]. The strength can be improved when the tempering temperature is even higher (600~650 °C) because large amounts of refined NbC particles precipitate in the tempering microstructure [33].

As the peak temperature increases to 900~1000 °C, the austenitization nearly completes. Nevertheless, the carbonitrides cannot dissolve, austenite grain size is very small and the alloy elements distribution is heterogeneous, which results that the higher phase transformation temperature is over 600 °C, even to 700 °C during cooling process (Figure 9), the higher phase transformation temperature results that the microstructure mainly consisted of PF and QF is obtained (Figure 8), so the lowest strength is obtained (Figure 3). With the increase in peak temperature to 1200~1300 °C, the carbonitrides begin to quickly dissolve, and the austenite grain size grows rapidly, so the stability and hardenability of austenite are improved. As a result, the phase transformation temperature during the cooling process decreases to below 600 °C, even to 525 °C (Figure 9), and then the final microstructures change to from a mixture of PF and QF to QF and BF (Figure 4); consequently, the strength increases (Figure 3). The results stated above indicate that the effects of peak temperature on the microstructure and strength of HAZ are attributed to the phase transformation during the weld thermal cycle, while the final microstructure is more related to the phase transformation during the cooling process. From this point of view, the strength of HAZ may correspond to the phase transformation. According to the results of the effect of peak temperature on strength (Figure 3) and phase transformation temperature (Figure 9), the effects of beginning phase transformation temperature on the strength of HAZs are summarized and shown in Figure 10.

From the results shown in Figure 10, it can be seen that the strength of HAZs seems to be strongly dependent on the beginning phase transformation temperature during the cooling process. Simultaneously, it can also be found that the chemical composition of steels has a significant impact on the beginning phase transformation temperature and strength of the HAZs. The strength of HAZ for the steels with lower content of C and alloying elements, i.e., lower values of Ceq, such as Steel D and E, is lower than that of the steels with higher values of Ceq, such as Steel B and C, which is attributed to the higher beginning phase transformation temperature obtained.

However, from Figure 9b, the chemical composition of the steels strongly affects the beginning phase transformation temperature of HAZs. At the same peak temperature, it decreases with the increase in values of Ceq of the steels. The increase in the amount of C and alloying elements can significantly enhance the hardenability of steels and lower the transformation temperature, consequently increasing the strength of the steels. Therefore, the control of the HAZ softening should consider the influence of alloy composition on the strength of the typical sub-HAZs. As shown in Figure 3, the lowest and highest strengths occur in typical sub-HAZs of CGHAZ and FGHAZ; thus, the strength relationship of typical sub-HAZs of CGHAZ and FGHAZ with the highest and lowest strengths, along with the values of Ceq of steels, are summarized and illustrated in Figure 11. The strength of all sub-HAZs seems to be linearly related to the Ceq of steels. Meanwhile, the minimum yield strength and tensile strength of X80 and X90 pipeline steels specified by API 5L are also marked in Figure 11. For the X80 pipeline steel, the minimum yield strength and tensile strength are 555 MPa and 625 MPa, respectively. The yield and tensile strength of HAZs after a weld thermal cycle at a peak temperature of 900 °C is the lowest, even lower than the 555 MPa and 625 MPa specified by the specification. The critical value of Ceq for strength below 555 MPa and 625 MPa are approximately 0.438% and 0.487%. The value of Ceq is close to 0.45% statistically analyzed by the X80 steel pipes reported in reference [31], which corresponds to the critical value of Ceq that the strength of the weld joints is higher or lower than the bodies. The result indicates that the softening of HAZs is the reason that the tensile strength of welded joints is lower than that of pipe bodies [31]. Whereas, the strength of the welded joints is affected by the strength and width of the softening zone [27], and the tensile strength of the welded joint must be higher than the minimum tensile strength of 625 MPa specified in the API 5L specification. From Figure 11, the tensile strength of all sub-HAZs of the steels with different Ceq is higher than the value of 625 MPa. This is why the strength of the weld joint of the X80 pipe can still meet the requirements of the specification of API 5L, even if the softening zone occurs in the HAZ.

In addition, from Figure 11, if for X90 pipeline steel, the softening zone in HAZs is further widened, it needs a higher value of Ceq to guarantee that the strength is higher than the minimum yield and tensile strength of 625 MPa and 690 MPa specified in the API 5L. For the FGHAZs with the lowest strength, the critical value of Ceq for strength over 625 MPa and 690 MPa is approximately 0.487% and 0.530%, respectively, even though the tensile strength of FGAZs is lower than 690 MPa, as the Ceq is lower than 0.442%. Meanwhile, for the CGHAZs with the highest strength, the yield strength is lower (690 MPa), as the Ceq is lower, 0.453%. The results indicate that the tensile deformation is mainly concentrated in the HAZ when the Ceq is lower than 0.453%; as a result, the softening of HAZ is more prominent. Thus, the higher content of C and alloy elements, i.e., Ceq value of pipeline steels, is necessary.

All results stated above, the softening zone may occur in the HAZ of high-strength pipeline steels, and the strength of the softening zone is strongly affected by the peak temperature and chemical composition of pipeline steels (Figure 3). Meanwhile, the effect of peak temperature and chemical composition on the strength is attributed to their influence on the phase transformation and final microstructure during the welding thermal cycle, especially the phase transformation during cooling. The strength of HAZ appears to have a linear relation with the beginning phase transformation temperature (Figure 10); meanwhile, the beginning phase transformation temperature strongly relates to the value of Ceq (Figure 9). Thus, at the same peak temperature, the strength of HAZ presents a linear relationship to the value of Ceq of pipeline steels (Figure 11). Therefore, in order to control the softening of the HAZ, the minimum value of Ceq for the high-strength pipeline steel should be an important parameter of pipeline steel.

5. Conclusions

In this work, the strength, phase transformation, and microstructure of the HAZ of six pipeline steels with different chemical compositions were studied, and the effect of chemical composition on strength and phase transformation was discussed. The main conclusions are as follows:

(1)

The strength of HAZs is significantly influenced by the peak temperature, and the softening zone mainly occurs in FGHAZ when the peak temperature is 900~1000 °C.

(2)

The peak temperature and chemical composition of the steels affect the phase transformation during the cooling process of the welding thermal cycle, the final microstructure, and the strength of the HAZs. The strength of HAZs is linearly related to the beginning phase temperature.

(3)

The strength of sub-HAZs at the same peak temperature is linearly related to the value of Ceq of steels. From the perspective of controlling the strength of the heat-affected zone, in order to ensure that the yield and tensile strengths of X80 and X90 pipeline steel are higher than the minimum values specified by API 5L, the values of Ceq of the steels should be over 0.438% and 0.487% for yield strength, and 0.399% and 0.453% for tensile strength, respectively.