Abstract
In the present study, we carried out a flux cored arc welding using a super duplex stainless steel as a filler metal and analysed the cause of cracking that occurred at the aging treatment for 30 min at 850°C. The stress concentration effect by the notch of the welding bead affected the hot cracking susceptibility, and the impact value of weldment was remarkably decreased due to the precipitation of secondary phases. In addition, the precipitation of σ of the adjacent area to the crack was measured to be higher by ∼12 wt- than the non-adjacent area to the crack. As a result of analysis of fracture plane, it was embrittled by segregation of impurities or inclusions, which acted as a starting point of the crack, and the crack grew according to the widely distributed σ phase.
Introduction
The duplex stainless steel (DSS) has high pitting corrosion resistance and stress corrosion resistance in sea water containing an anion such as Cl− as well as a chemical plant industry that handles the solution of sulphate ion. Thus, it is widely used in the marine structure and chemical plant industry to the generation plant facility.1–3 However, as industrial technology develops, the material available under a more severe environment was required, and one of the developed materials by such a demand is super duplex stainless steel (SDSS) having the superior characteristics to the existing DSS. The SDSS has >40 of pitting resistance equivalent (PREN), 4 and the corrosion resistance and mechanical properties are superior to the existing DSS.5,6 Thus, the usage range of SDSS is widely extended in various industrial sectors, which require high/low temperature characteristics as well as severe corrosion environment.
The SDSS adds the considerable amount of alloying elements such as Cr, Mo and Ni for the improvement of corrosion and mechanical properties. Owing to the effect of these alloying elements, the ferrite is changed into a different phase at a relatively wide temperature section (300–1000°C) while processing or using at a high temperature, and as a result, the volume fraction of two phases of δ-ferrite and austenite (γ) is changed and the characteristic of materials is lowered.7,8 At this time, the weldment accompanies the severe thermal change. Especially, in case of exposure at a high temperature of 600–900°C, the formation of weak secondary phases such as σ and χ is promoted in the alloy. Especially, for the α phase, even if a very small amount is precipitated, it greatly lowers the impact toughness of the material.9,10 Thus, to apply the developed alloy widely to the industrial site, it is required to inhibit the formation of secondary phases such as the σ and χ phases at maximum at the weldment including the heat affected zone after welding, not to lower the impact toughness and crack resistance.
As the crack that occurs in the weldment is very complicated and appears in various types, there are several classification methods. The crack is classified by occurrence place, occurrence direction, occurrence temperature, occurrence type and occurrence cause. The hot cracking occurring mainly in the stainless steel occurs if the structure, which is sensitive to the strain and crack by self-shrinkage and external binding at a high temperature during welding or just after welding, exists. The feature of hot cracking is as follows. 11 The crack is the intergranular crack that occurs mainly in the grain boundary, and the end of the crack shows the curve type and not the straight type. In addition, the crack area exposed on the surface is oxidised with colour by air, but the concealed area is not oxidised with a silver white colour.
As the cause analysis of the damage of weld metal, that is, the crack occurrence is carried out to prevent the reoccurrence of the same crack, it is important to establish the countermeasures considering the current business. The present study looked into the cause of crack and various factors that give the effects to the crack propagation when the weld metal was exposed to an extreme temperature of 850°C, enough to be brittle by the crack for 30 min, after carrying out the flux cored arc welding (FCAW) using the SDSS steel as filler metal.
Experimental
Welding materials and welding method
In the present study, buttering treated SDSS was used, as shown in Fig. 1. This means that each groove was buttered up to 3 mm with the same filler wire in the present study to minimise the dilution between the AISI 304 austenite SS and SDSS weld metal, so the buttering layer was not considered in the investigation. The FCAW welding was carried out in the state of facing with AISI 304 with 290×140×20 mm thickness. As shown in Fig. 1, the route gap of 10 mm and the included angle of 45° were used, and for the protection gas, 25 L min−1 of 99·9CO2 was used and the DCRP(+) was used for the current. The details of the welding parameters are shown in Table 1.
Schematic diagrams of weldment
Welding conditions*
*Shielding gas/flowrate: CO2 (100), 20–25 L min−1, polarity: DCRP(+). Electrode extension (mm): 15–20.
Test specimens were received in the form of a plate of 20×15 mm and 5 mm in thickness in welds, and their chemical composition was measured using an optical emission spectrometer (Metal-Lab75/80J, GNR srl, Italy), and the value is shown in Table 2. For the specimen collected by cutting as shown in Fig. 1, water quenching was carried out after aging treatment for 30 min at 850°C, and the cause of longitudinal cracking detected at the weld metal of specimen was analysed.
Chemical composition of weld metal*/wt-
*PREN30 = Cr+3·3(Mo+0·5W)+30N. Creq = Cr+Mo+1·5Si+1·5Nb. Nieq = Ni+30C+0·5Mn.
Microstructure observation and phase analysis
To observe the microstructure of weldment and precipitate, we carried out electrolytic etching using two types of etching solutions according to the characteristics of observation after grinding and polishing. To observe the general γ and δ-ferrite, 10 of oxalic acid was used, and for the observation of abnormal precipitate, 10KOH solution was used and the precipitate such as the σ phase was seen with grey colour. In addition, to analyse the microstructure, the scanning electron microscopy (SEM) (JSM-9400F, JEOL, Japan) and energy dispersive X-ray were used, and through the phase analysis by electron backscattering diffraction (EBSD), the cause of the crack was analysed.
Impact test
After locating the notch of the impact test specimen in the centre of weldment, we machined 2 mm from the surface of the weld face area and manufactured the specimen based on ASTM E23-02 (Standard Test Methods for Tension Testing of Metallic Materials). The notch direction of the impact test specimen is parallel with the welding direction. The impact test temperature was calculated for the average value after carrying out three times each at −20, 0 and 20°C based on classification criteria. The shape of the specimen is as shown in Fig. 2.
Standard Charpy impact test specimens
Results and discussion
Effect of shape of weld bead
The crack showed the feature of general hot cracking like in Fig. 3. The crack is the intergranular crack that occurs in the α/γ grain boundary, and the end of the crack is the curve type, not the straight one. The exposed area on the surface of the crack fracture plane was oxidised by the air, and the concealed area was not oxidised with a silver white colour.
Schematic diagram of weld cracking
The hot cracking occurs if, at the high temperature just after welding, the structure sensitive to the strain and crack by self-shrinkage and external binding exists at the weldment at the high temperature just after welding. 12 As the weld metal is cooled locally after rapid heating, the restraint stress always exists. Thus, when selecting the welding material or constructing the welding, it is essential to do the welding design that the restraint stress is applied less so that the resistance force for the crack at high temperature can be bigger.
Another important factor that affects the hot cracking of weldment is the shape of the weldbead. The shape of the weld bead affects the binding degree that acts to the weldment at the end of solidification. Figure 4 is the mimetic diagram of the occurred crack, and the crack occurred equally from the local notch area of the weld bead. For the specimen of aging treatment after grinding the notch, the frequency of the crack occurrence is reduced in the same condition.
Features of hot cracking
It is known that to improve the fatigue life of the weld metal of the structure and the area that the fatigue damage is expected, the weld bead or spatter is trimmed by cutting or grinding, which is effective to improve the fatigue strength. This results in the effect to remove the defects of weld metal and reduce the stress concentration by changing the bead shape. 13 Therefore, the post-treatment that machines the weld bead smoothly after welding will be effective to improve the fatigue life of weld metal and the crack resistance at high temperature.
Effect of precipitates
To find the correlation of mechanical characteristics and microstructure, we analysed the microstructure with an optical microscope. As shown in Fig. 5, we used 10KOH etching solution to show the precipitates such as the σ phase with dark grey colour. At the aging treatment, the reaction of the δ ferrite→σ+γ2 became active, the δ-ferrite in the metastable phase has been transformed into the σ phase and the crack was propagated along with the α/γ interface. Through EBSD analysis, the microstructure of seven-pass areas was analysed, which is expected as the starting metal of the same crack for all specimen. As a result of phase analysis through the EBSD method, as shown in Fig. 6a, the specimen in the as weld state had a 6∶4 volume fraction of the γ and δ ferrite, and the secondary phase precipitated by the weld heat was not detected. On the contrary, as shown in Fig. 6b, the specimen of the aging treatment for 30 min at 850°C showed the active transformation into the χ and the σ phases by the diffusion of lots of Cr in the δ ferrite, which reduced the ferrite from 41 wt- before aging treatment to 14 wt-.
Microstructures of weld cracking
Electron backscattering diffraction phase map (colour scheme for phase ID: red, γ-austenite; green, δ-ferrite; yellow, sigma phase; blue, chi phase)
Table 3 is the result of the Charpy impact test. The impact value for the FCAW weldment was remarkably reduced due to the precipitation of the secondary phase and did not meet the impact absorption energy value (34 J) required by classification criteria for all temperature.
Charpy impact energy of service exposed pipe (size of specimens 5×10×55 mm)
Deng et al. 14 found that the σ phase having the fast formation kinetic was formed the most at 850°C to improve the hardness and reduce the impact toughness remarkably.
Figure 7 is the EBSD phase map of the non-adjacent area to the crack (Fig. 7a) and the adjacent area to the crack (Fig. 7b). According to the difference of surface roughness, the crack area data having low confidence index were excluded. As a result of analysis of precipitation of two areas, the σ phase of the crack adjacent area was measured to be ∼12 wt- higher than the non-adjacent area.
Electron backscattering diffraction phase map (colour scheme for phase ID: red, γ-austenite; green, δ-ferrite; yellow, sigma phase; blue, chi phase)
For the specimen of the aging treatment at 850°C, which is the sensitivity temperature of the secondary phase precipitation, it is judged that the σ phase that precipitated in the α/γ interface in advance occupies the δ-ferrite and grows, and the impact toughness was reduced and, accordingly, the hot cracking susceptibility became very high, which caused the crack to propagate.
All cracks occurred only for the specimen that had the aging treatment additionally after welding not the specimen just after welding. This means that the secondary phase was not extracted by the weld heat during welding, but the secondary phase was extracted rapidly by the additional aging treatment, which caused the crack. Thus, in order to prevent the crack, it is required not to use the weld metal at 700–850°C easily to extract the sigma phase or reduce the use time at maximum.
Effect of segregation of non-metal inclusions
As a result of analysis for the fracture plane of the impact test specimen, the particles of inclusions of 2000–5000 Å were found in the centre of the dimple as shown in Fig. 8. As a result of component analysis, Mn and Si were detected. This means that some of the melting type exists at the crack metal finally and the low melting point inclusion is in the state of melting under the welding temperature due to the decrease in melting temperature by the increase in C volume at the segregation, which remain in the process of pressing. In addition, the component, such as Si, which looks as inclusion, is the element to be added during steel manufacturing, and this inclusion plays the role of source of low melting point oxide on the basis of crack.
Segregation and inclusions present in impact test specimen fracture surface of weld
As a result of analysis for the fracture plane of the crack as shown in Fig. 9, the particles of 2000–5000 Å were also found in the fracture plane. Since in the weldment, due to the fast weld heat cycle in the process of solidification after melting basic materials, there is not enough time for lots of Cr and C to be homogenised, and the segregation of Cr and C occurs in the grain boundary and dendritic boundary, which will finally be solidified. When the materials are destroyed, the stress concentration will be generated by these particles, and due to the stress concentration, the transformation will increase locally and the crack will begin. However, the big inclusions reduce the soft properties of materials and determine the starting moment and location of the crack only, and the proceeding of crack will be influenced by the secondary phase precipitated in the process of aging. 15 It is judged that the precipitates and inclusions act as a cleavage fracture path. In the correlation of microstructure and mechanical properties, the growth and development of inclusions and segregation brought the result that reduces the impact toughness of weld metal.
Segregation and inclusions present in crack fracture surface of weld
When the weldment is exposed to a high temperature after welding, Cr and Mo will be cohered excessively in the α/γ interface, and if the secondary phase is precipitated, the surroundings will become a defective area due to the deficiency of Cr or Mo. The defect may cause the concentration of stress locally, from which the crack will begin, and as the number of defects increases, the possibility of the crack occurrence will be high. It is known that if the secondary phase, such as the σ phase or inclusions, is precipitated in the α/γ interface, it may facilitate the generation and propagation of the crack to reduce the toughness of the alloy. 16 In addition, for the stainless steel containing approximately (22–28)Cr− and (3–5)Mo, it is reported that the σ phase will form Fe−(29–31) and Cr−(6–8Mo).17–19 As shown in Fig. 10, the segregation and inclusions were observed everywhere in the boundary, and lots of cracks were propagated by surrounding these inclusions. If the segregation or inclusion exists in the material, this can be acted as stress concentration source and the crack can be occurred and propagated based on this. 20
Image (SEM) of cracks grown to around inclusions
Conclusions
The major conclusions from the present study are the following.
As the crack occurred equally from the local notch area of weld bead equally at all specimen, the effect of stress concentration by weld bead notch affected the hot cracking susceptibility.
As a result of analysis for the fracture plane of the impact test and the crack fracture plane, the particles of 2000–5000 Å existed, and due to these particles, the stress concentration was generated and the transformation by the stress concentration increased locally and the crack began. In addition, the segregation and inclusions were observed everywhere in the boundary, and the large numbers of cracks were propagated by surrounding the inclusions.
Owing to the segregation of impurities and inclusions, SSD was embrittled first, which acted as the starting of the crack and the crack grew along with the widely distributed sigma phase. It is judged that as the number of such defects increased, the crack grew.
As a result of analysis for EBSD phase map, the precipitation of the σ phase of the adjacent area to the crack was ∼12 wt- higher than that of the non-adjacent area.
The impact value for FCAW weldment was remarkably reduced due to the precipitation of the secondary phase and did not meet the energy absorption energy value (34 J) required by the classification for all temperatures.
Footnotes
Acknowledgement
The present research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation.