Influence of flame straightening on the properties of welded joints made of X2CrNi22-2 duplex steel

An important advantage of duplex steel in comparison with traditional austenitic steels is its lower content of expensive nickel and superior strength properties, in addition to its preservation of good deformability and corrosion resistance, including in environments containing chloride ions [1]. However, their limitation is their tendency for brittleness after annealing within the temperature range of 280–520°C (the so-called 475°C brittleness), which can appear after just several minutes of heat application, as well as sudden plasticity reduction caused by precipitation of the sigma and chi intermetallic phases, occurring within the temperature range of 610–950°C [2].

The mentioned adverse changes in ferritic and duplex steels also occur under the influence of welding heat. Therefore, it is justified to ask whether heating with an acetylene-oxygen flame during straightening of constructions made of these steels has a negative influence on the properties of welded joints, all the more so since the studied steel contains molybdenum.

With regard to flame straightening of constructions made of stainless steels, it is decidedly advisable not to apply such treatment for straightening of austenitic chromium-nickel steels [1]. However, there is no such indication with respect to ferritic and duplex steels. Moreover, analysis of the literature shows that flame straightening of duplex steels is practiced [3]. In Germany, training seminars on flame straightening of various constructions, including those made of duplex steel, are conducted [3]. Linde [4], a manufacturer of technical welding gases, also manufactures equipment for flame straightening, and its guidelines state that flame straightening of high-alloy austenitic steels generally leads to structural changes in addition to surface oxidation. However, the surface does not undergo change when the flame temperature is maintained between 550°C and 750°C (“dark red”) during straightening. Carburization is avoided with a neutral flame, and even with excess oxygen. Due to lower thermal conductivity and greater thermal expansion, heat accumulation and rapid action are achieved; and because of this, smaller torch nozzles are applied than in the case of structural steels. Rapid cooling has a beneficial influence on material quality and steel corrosion.

The possibility of subjecting duplex steel to flame straightening is assumed based on the following facts: (i) the grain growth observed in the high-temperature HAZ (Heat Affected Zone) zone (near the fusion line) occurs at high temperatures (>1,000°C), and such temperatures typically do not occur during straightening; and (ii) the process of intermetallic phase precipitation depends not only on temperature but also on heating time. It turns out that, for the linear energies applied during welding, the joint's cooling rate is greater than the critical rate of sigma phase formation [5]. As indicated in the chart presented in Xavier et al. [5] and as other studies have confirmed, the sigma phase should not precipitate.

However, the start and end times of the sigma phase precipitation process also depend on the chemical composition of the steel. A comparison of grade 2205 and 2507 duplex steels [6] shows that the sigma phase appears in 2205 grade steel after 1 h of heating at 800°C, while in grade 2507 steel, the sigma phase appears after just 6 min of heating at 700°C. Meanwhile, the process of chromium carbide and chromium nitride precipitation begins within a relatively short time of 1–2 min. The authors of the publications state that the kinetics of chromium carbide and chromium nitride precipitation for all duplex stainless steel alloys with nitrogen are similar to that for 2205 grade steel [6].

The most important intermetallic phases during production and welding of duplex steel are sigma, chi, secondary austenite, and chromium nitrides. All of these phases form at temperatures >500°C. When the temperature is ≤ 500°C, precipitation reactions are relatively slow, and thus the influence exerted on brittleness is small [7].

The literature shows that the fastest rate of sigma phase formation from ferrite takes place at a temperature of ≈ 750°C, and weld metals containing molybdenum exhibit the greatest tendency to form the sigma phase, followed by those containing silicon, tungsten, vanadium, and niobium, in order of decreasing intensity [8].

In order to limit heat efflux into the neighboring material (which should remain cool) and avoid inclusions of intermetallic phases (including the σ phase), the material should be heated within the shortest possible time [9]. The effect of annealing time on σ phase formation is addressed in the literature in a 1991 study [10], and also in a guide published by Linde [6].

Due to the risk of adverse effects occurring in the material during performance of the flame straightening process, this method is applied very rarely and only when necessary [11]. Duplex steel is used in the construction of means of transport, e.g., for vehicle frames. The large number of joining elements and welds promotes welding distortion in such structures. Flame straightening could be a cheap and quick way of straightening them. Further interesting conclusions that can be taken into consideration with regard to flame straightening of duplex steels are available in Łabanowski et al. [12]. The effect of heat treatments, including the resultant changes in microstructure and mechanical properties, mainly impact toughness, on duplex stainless steel and higher alloy superduplex steels is discussed in the study by Topolska and Łabanowski [13]. In their 2019 study [14], Golański and Lachowicz recognize the sigma phase present on ferrite grain boundaries as the reason for the brittle rupture of the suspension. An excellent description of the influence of heat on the presence of the sigma phase is available in the 2009 study by Garin and Mannheim [15]. The latest research confirms the possibility of testing with non-destructive techniques for the detection of the sigma phase in duplex stainless steel [16], which is difficult, however, because it requires appropriate preparation of the sample for testing. Moreover, despite the many available methods, the identification of the sigma phase in duplex steels is still a difficult issue [17]. It is true that sigma phase detection in duplex stainless steel is possible with an induced magnetic field [18]; however, it is rather difficult to detect the moment of its appearance in duplex steel when the sigma phase occurs in very small amounts.

The subjects of the present research were (i) thermally untreated welded joints, which were designated by the letter S; and (ii) joints heated by flame to a temperature of 730°C, intended to simulate the heating occurring during flame straightening, which were designated by the letter W. The joints used were butt-welded joints having closed profiles of 30 × 30 mm, made of corrosion-resistant ferritic-austenitic steel of the grade X2CrNiN22-2. Figure 1 shows a view of the joints. Joints not subjected to heat treatment after welding were marked DS (Duplex Steel); and welded joints annealed after welding at 730°C were marked DSA (Duplex Steel Annealed). The joints were annealed for periods of 15 min, 5 min, 2.5 min, and 1 min, after which they were cooled in air. Samples were extracted from all joints for testing.

Fig. 1

The scope of research included the following:

Macroscopic observations of welded joints made with the unaided eye, and with a SMT-800 stereo microscope and a Neophot 32 light microscope at 20× magnification.

The tensile strength test of joints was performed on non-standard samples. Microscopic examinations of welded joints were conducted on cross-sections of welds in the native material, in the heat-affected zone near the fusion line and weld, and under magnifications ranging from 50× to 500×; the observation was carried out in the unetched state and then again after etching with Mi30Fe reagent according to the standard PN-H-04503:1961.

An OLYMPUS CK40M metallographic microscope, coupled with a CCD digital camera with image recording, was applied. Chemical composition analysis was performed in an argon atmosphere on a LECO GDS-750-QDP glow discharge spectrometer.

2.1

Chemical composition analysis

Samples for chemical composition studies were sourced from the 1.4062-grade duplex steel, and extracted from the weld. Figure 2 shows the samples used for the chemical composition analysis, and Table 1 shows the results of the analysis.

Fig. 2

Table 1

Results of the analysis of the chemical composition of tested materials

Cont.% by weight	Steel grade 1.4062 (X2CrNi22-2)		Wire G 22 9 3 N L	Weld in the joint 1.4062+1.4062
	Certificate No. 51513524	Analysis results by LECO	Certificate No. 102004	Analysis results by LECO
C	0.020	0.025	0.1	0.025
Si	0.450	0.30	0.5	0.41
Mn	1.340	1.33	1.7	1.68
P	0.023	0.030	0.016	0.030
S	0.001	0.015	0.001	0.013
Cr	22.800	22.05	22.9	22.60
Ni	2.640	2.46	8.8	7.68
Mo	0.210	0.19	3.1	1.86
Nb	–	0.03	–	0.03
Cu	–	0.23	<0.1	0.017
N	0.216	–	0.16	–
Co	–	0.05	–	0.06
V	–	–	–	0.07
W	–	–	–	0.01

Based on the results of the analysis of the chemical composition of the materials tested, it can be concluded that there was a low degree of mixing of the filler metal components with the steel components in the welded joints. This may be the result of shallow fusion of the weld metal into the joint parent material. The content of elements in a welded joint is very similar to the chemical composition of the alloy of the filler metal used.

2.2

Metallographic examinations

Microscopic examinations of native materials in the unetched state revealed a small amount of non-metallic inclusions, mainly in the form of oxides distributed in points.

After etching with Mi19Fe reagent, a fine-grained ferritic-austenitic structure with diverse grain size could be observed (Figure 3), with high intensity of banding, as typical for materials subjected to metalworking (rolling).

Fig. 3

The microscopic tests conducted in the vicinity of the fusion line of grade X2CrNiN22-2 duplex steel, in both butt welds S and W, revealed a ferritic-austenitic structure with expanded and re-crystallized grains of ferrite and austenite, with visible σ phase clusters rich in Cr and Mo, forming in the area of ferrite grains and on the ferrite-austenite boundary (α/γ), as depicted in Figures 4–13. In the joint welded and soaked at 730°C (W), a rather substantial grain growth was observed, with a very large amount of intermetallic σ phase rich in chromium (Cr) and molybdenum (Mo). An aci-form, dendritic, ferritic-austenitic structure was observed in welds, as depicted in Figures 14 and 15.

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11

Fig. 12

Fig. 13

Fig. 14

Fig. 15

It should be mentioned that the sigma phase (σ) rich in Cr and Mo may precipitate in the 650–1,000°C temperature change within the area of ferrite grains or on the ferrite-austenite boundary (α/γ) during the welding process, or may form as precipitations of secondary phases during heat treatment. This phase forms from ferrite, and causes depletion of adjacent areas of Cr and Mo, reduction in resistance to pitting and intercrystalline corrosion, and increase in brittleness. Elements such as chromium (Cr), molybdenum (Mo), manganese (Mn), silicon (Si), and tungsten (W) foster the formation of the σ phase [1].

Tests of the kinetics of sigma phase formation during soaking of type 18-8 weld metals at 750°C revealed that ≈ 70% of delta ferrite degrades into the sigma phase and ≈ 30% into secondary austenite, and this degradation typically begins after just 30 s [3].

The W welded joint was made with an excessively high face excess and large root leaks, which indicates a large supply of heat. Thus, it was not possible to precisely determine whether sigma phase precipitations formed as a result of the welding process or the additional heat treatment, which is why it was decided to soak the other parts of S samples at 730°C for 15 min, 5 min, 2.5 min, and 1 min in a silit rod furnace without a protective atmosphere, with cooling in air, and to conduct additional microscopic observations.

The conducted microscopic examinations revealed that in the DS and TS samples, in the heat-affected zone in the X2CrNiN22-2-grade duplex-steel material (designated No. 1.4062), during soaking at 730°C over the course of 15 min, 5 min, and 2.5 min, and cooling in air, a greater amount of sigma (σ) phase precipitated in ferrite and on the ferrite-austenite (α/γ) boundary than in the same sample prior to the soaking process (Figures 16–19); and in the sample soaked at 730°C for 1 min and cooled in air, no sigma (σ) phase precipitations were identified in ferrite or on the ferrite-austenite (α/γ) boundary (Figure 20).

Fig. 16

Fig. 17

Fig. 18

Fig. 19

Fig. 20

2.3

Hardness measurements

Hardness measurements of welded joints were performed using the Vickers method, according to PN-EN ISO 650: 2007. A Zwick 321 hardness tester was applied with the penetrator under a load of 9.807 N, acting for a time of 15 s. The system of hardness measurement points on the metallographic specimen of the joint is shown in Figure 21.

Fig. 21

The hardness of the native material, duplex steel, was 295 HV1, the hardness of the joint without heat treatment after welding in the heat-affected zone near the fusion line was 229 HV1, and the hardness of the joint was 248 HV1; further, the hardness of the joint heated with a flame to ≈ 730°C amounted to 222 HV1 and 245 HV1 in the HAZ and in the weld, respectively. Hardness change in the tested joints is presented in Figure 22.

Fig. 22

The decrease in hardness in the heat-affected zone is mainly the result of recrystallization of the material's structure, which was in a state of cold work after rolling. The results of the conducted hardness measurements indicated that the process of soaking the welded joints at 730°C in the HAZ near the fusion line did not cause significant differences in hardness. The small hardness reduction visible in Figure 22 results from further recrystallization of the material's structure during soaking.

2.4

Tensile strength evaluation

Three flat specimens in the form of strips were collected according to PN-EN ISO 6892-1, Appendix E, for tests from every sample group. The results of strength tests are shown in Table 2, and the appearance of specimens after the tensile test is shown in Figure 23.

Fig. 23

Table 2

Results of static tensile test of specimens of butt welds without heat treatment after welding (S samples) and butt welds heated with acetylene-oxygen flame to ≈ 730°C (W samples)

Item no.	Designation of joint specimen	a×b (mm)	S0 (mm2)	Fm (N)	Rm (MPa)	Rm avg (MPa)	Rm (MPa) according to PN-EN 10088-2
1.	Sb	19.94 × 1.98	39.5	2,6600	673*	727	700–900
2.	Sc	19.94 × 1.94	38.6	28,400	736
3.	Sd	19.94 × 1.96	39.1	30,200	772
4.	Wb	19.95 × 1.91	38.1	27,200	714	739
5.	Wc	19.8 × 1.89	37.4	29,400	786
6.	Wd	19.86 × 1.91	37.9	27,200	718

*

Welding non-conformity occurring in weld (partial lack of melting).

Based on the performed strength tests of butt-welded joints without heat treatment after welding and joints heated by acetylene-oxygen flame to ≈ 730°C, it was determined that fracture in the specimens occurred in the weld.

The tested joints were made with one bead, which resulted in a low tendency for sigma-phase precipitation in the HAZ near the fusion line and in the weld. The appearance of the sigma phase in such a configuration does not have a significant influence on the strength of welded joints. Even the appearance of a small amount of sigma phase in root layers of multi-layered welds does not significantly reduce the strength and plasticity properties of welded joints made of type 18-8 steel [3].