Low-cycle Fatigue and Fracture Behaviour of 9%Ni Steel Flux Cored Arc Welding joint at Cryogenic Temperature

. In the present work, low-cycle fatigue (LCF) test and crack tip opening displacement (CTOD) test were performed for 9% Ni steel flux cored arc welding (FCAW) joint at room temperature (296 K) and cryogenic temperature (80 K). At cryogenic temperature, the strain amplitude had a far greater impact on fatigue life of 9%Ni steel welded joint and it decreased dramatically lead to a significant increase in fatigue life. It was found that most fracture initiation of joints located in fusion area at room temperature, while it occurred in weld seam at low temperature. The fracture toughness of weld seam was higher than that of fusion zone no matter the testing temperature. The effect of precipitated phase was the true reason. The fatigue cracks propagated in transgranular mode at room temperature, ultimately, and intergranular mode at low temperature in both LCF specimens and CTOD specimens.


Introduction
In recent years, performance of cryogenic material has gained more attention than before. In most cases, the construction of 9%Ni steel welded structures service aggressive environments with fluctuating load and cryogenic temperature. Its cryogenic fatigue property and fracture toughness were critical to product safety. Currently, with good efficiency and adaptability the Flux Cored Arc Welding (FCAW) was used for 9%Ni steel in LNG tanks and showed good performance [1]. However little research has been done on fatigue property of 9% Ni steel welded joint. The microstructure and performance of 9%Ni steel joint made by FCAW are still not investigated fully.
Ni-based alloy has been the preferential candidate for 9%Ni steel welded structures [2]. Studies in Ni-based alloys, while plentiful, are predominantly focused on fatigue and fracture property at room and high temperature [3][4][5][6][7]. And yet, very little research has addressed that at cryogenic temperature. The weld metal mainly consists of austenite and precipitates. Some researches were carried out on the effects of precipitates in the weld metal of nickel based alloy. [8] studied the deposited metal of Ni-Cr-Mo-Nb alloy in TIG cold wire welding. They identified that the precipitated phases were Laves phase and complex carbide particles. [9] investigated high-cycle fatigue (HCF) properties at 4K, 77K and 293K in forged-INCONEL 718 Nickel-based alloy. It was found that fatigue cracks predominantly initiated from coarse Nb-enriched MC carbides which seem to lower the high-cycle fatigue strength at cryogenic temperature. [10] investigated the effects of precipitates on at -196℃.They found that Laves phases provided a preferential site for crack initiation and propagation. The performance degradation of weld metal had close relationship with the size and volume percentage of Laves particles. [11] found that the carbide located at grain boundary, increased noticeably the incidence of cracks. From this point of view, influences of precipitated phases deserve more attention on the study of fatigue property.
In this paper, the fatigue property and fracture toughness of welded joints of 9%Ni steel by FCAW under cryogenic environment were studied. Low cycle fatigue test was used to study the fatigue property of the joint at room temperature (296 K) and at cryogenic temperature (80 K).
It was found that the weakest position in the joint was the fusion area at room temperature while weld metal at cryogenic temperature. As a consequence, the fracture toughness of these positions were investigated by using CTOD test. The crack propagation mode in the stable propagation region of LCF and CTOD specimens was observed. The effect of cryogenic environment on the LCF and fracture toughness were invested in the paper.

Material and welding procedure
The composition of 9%Ni steel (by QT) and filler wire are listed in Table 1. The base metal plates were processed into sizes of 400 mm (L) × 200 mm (W) × 20 mm (H). The plate were machined into K-groove configuration and welded along transverse-to-rolling direction by flux cored arc welding (FCAW) processes. The flux-cored wire is DW-70S, the diameter is 1.2mm. The welding parameter was listed in Table 2.There were two welding passes in top part and two passes in back part, respectively. The interpass temperature was controlled below 100℃ strictly for all welding processes. The welded joints have met the standard by nondestructive X-ray examination.

Low cycle fatigue test
In low cycle fatigue test, uniaxial fatigue tests were carried out using MTS 370 fatigue testing system. cylinder specimens with 25 mm gauge length and 8 mm diameter were prepared as shown in Fig 1. According to the test standard [12]. Uniaxial strain-controlled was employed. A triangular waveform was selected and the strain ratio was R = -1.0. The strain rate was 3.0×10-3 s -

1
. Different strain amplitudes were chosen from 0.3% to 0.8% until the specimen fracture or the maximum load decreases 50%. Coffin-Manson relationship was used to characterize the low-cycle fatigue behavior [7]. According to the theory, the total strain amplitude consist of the elastic strain amplitude and plastic strain amplitude in cyclic loading: ∆ε t 2 ⁄ = ∆ε p 2 ⁄ + ∆ε e 2 ⁄ (1) Where σ f ' is the fatigue strength coefficient, E is the Young's modulus, b is the fatigue strength exponent, ε f ' is the fatigue ductility coefficient, c is the fatigue ductility exponent and N f is the number of reversals to fatigue failure.
Ramberg-Osgood equation is used widely to describe the cyclic stress-strain relationship [13]: Where ∆σ 2 ⁄ is the mid-life stress amplitude, ε p /2 is the mid-life plastic strain amplitude, K ′ is the cyclic strength coefficient and n is the cyclic strain hardening exponent.

CTOD test
In precracking process, the precracking frequency was 10Hz and the maximum fatigue stress intensity factor applied during the final stages of fatigue crack extension was 25Mpa/mm 2 . It was conducted at room temperature.
The notches of CTOD specimens mainly located in weld seam and fusion zone. As shown in Fig 1, the initial crack location of weld seam specimen was the center of the third weld pass (Point A), while that of fusion zone specimen was the fusion line corrispond to the third weld pass (Point B). In three-point bending process, displace control mode with 0.5mm/min was used. In the process conducted a clip-on displacement gage was used to measure the opening displacement of the notch. According to the standard in the BS 7448 standard [14,15]. The crack tip opening displacement δ is composed of elastic displacement and plastic displacement ie: Where, W and B are the width and the thickness of specimen, z is the blade thickness, F is the load, S is the All tests were performed isothermally at 296 K and 80 K. When test was conducted at low temperature, advanced environmental chamber the temperature range of which could be from -196℃ to 350℃. The size of the chamber was 230mm (L) × 230mm (W) × 340mm (H). Liquid nitrogen was to adopt to hold cryogenic temperature constant in environmental chamber.

LCF behaviour
The fatigue life of joints with different total strain amplitude were presented in Fig 2. It was found that under the same total strain amplitude, the fatigue life increased dramatically with the temperature decreased from 296 K to 80 K. With the total strain amplitude increased, the increasing amplitude of low-temperature fatigue life reduced ultimately. It dropped from 5.26 times to 0.85 times at 80 K with the increase of the total strain amplitude from 0.3% to 0.8%.
The location of the fracture initiation of all joints were observed. It was found that most fracture initiation located in fusion area at room temperature (Fig 3(a)), while it occurred in weld seam at cryogenic temperature as shown in Fig 3(b). It was inferred that the weakest position in the joint was the fusion area while weld metal at cryogenic temperature.  Table 2 by linear fitting. It was clear that the cyclic strength coefficient K' and the cyclic strain hardening exponent n' of the joint were both larger at 80 K than that at 296 K. It was indicated that when the plastic strain amplitude was the same, the strength of the joint at low temperature was larger leading to the better low cycle fatigue property. It was clearly seen that the absolute values fatigue ductility coefficient ε', fatigue ductility exponent c, fatigue strength coefficient σ', fatigue strength exponent b at cryogenic temperature are greater than that at room temperature, which reflecting quite different plastic and elastic deformation characteristics of joints at different temperature, which could be further explained by the microstructure of different zones. K. It was found that the stress amplitude increased with increasing total strain amplitude. Under the same total strain amplitude, the load peaks at cryogenic temperature was higher than that at room temperature. The enclosed area was determined by strain amplitude and stress amplitude. Combined with the distribution of elastic and plastic strain amplitude as shown in Fig 4(c) and Fig  4(d). The elastic strain amplitude was the main reason leading to the higher axial stress at cryogenic temperature. Due to higher plastic strain amplitude at room temperature, the enclosed area of the hysteresis loops at 296 K was much larger than that at 80 K. Plastic strain energy at cryogenic temperature was far less than that at room temperature, resulting in the increase of fatigue life at cryogenic temperature.  . 4. The cyclic stable hysteresis loops with a total strain amplitude of (a)0.40% , (b) 0.60% at 296 K and 80 K

Fracture toughness distribution
Based on different fracture locations in LCF test, it is necessary to study the fracture toughness of both areas in the joint by CTOD test. Fig 5 shows the CTOD values of both areas at 296 K and 77 K. It was found that the fracture toughness, represented by the CTOD values, of all regions had a big drop when the temperature drop to 80 K. It decreased 41.9% in weld seam and 6.4% in fusion zone respectively. The CTOD value of weld seam was 74.0% higher than that of fusion zone at 296 K, and it was 7.9% higher at 80 K. It could be inferred that initial cracks located in fusion zone propagate much easier due to low fracture toughness at room temperature. However at low temperature the fracture toughness in weld zone was very close to that in fusion zone. There are some reasons else lead to the fracture location in weld seam at low temperature. The max force of weld seam specimen was lower than that of fusion zone specimen. The max force of specimen tested at 296 K was lower than that at 80 K. The maximum value of notch opening displacement corresponding to max force was composed of two parts: the plastic part and the elastic part. It was clear that the elastic component increased when temperature dropped into 80 K. It was because that the yield strength of materials increases at low temperature. it takes more force to produce plastic deformation of sample. Different from the elastic part, the plastic part of COD varied between two regions. The COD'S plastic component of weld seam was higher than that of fusion zone both at room temperature and in cryogenic environment.  The growth direction of austenite is along the temperature gradient. The average number of laves phases was computed by counting 3 zones of 0.1mm×0.1mm size randomly in appropriate areas (Fig 7  (a), Fig 7 (b)). The statistical results were 2.06×10 4 /mm 2 and 1.32×10 4 /mm 2 in fusion zone and weld seam, respectively. The number of fusion zone was 84.6% higher than that of weld seam. As shown in Fig 8 (a), the bright area is dendritic cells boundary, while the dark area is in dendritic cells. The precipitated phases mainly distribute in dendritic cells boundary. By EDS line-scanning, it was observed that the content of Mo element in dendritic cells boundary is more than that in dendritic cells and the content of other elements (Cr, Fe, Ni)in dendritic cells boundary are less than that in dendritic cells. The elements segregation is helpful to the formation of precipitated phase. EDS analysis also carried out to identify the types of precipitated phases as shown in Fig  8(c) and Fig 8(d). Relating to the out early research [16], it was sure that the precipitated phase conclude laves phases, which is always rod-shaped and rich in Nb and Mo.

Crack propagation behavior
Fig 9 shows the fatigue crack propagation path of LCF specimens tested at 296 K and 80 K. At room temperature, most cracks propagated through dendrite grain, as shown in Fig 9 (a). The propagation path was straighter. Few cracks were observed to propagate along the dendrite boundary. Fig 9 (b) presented the crack propagation paths in cryogenic temperature. Most cracks passed through the grain boundaries even though there were only a few precipitates, meaning that the intergranular propagation mode was dominant. Crack passing through the precipitate phase became a priority at cryogenic temperature. As shown in Fig 11 (b), microvoids were observed in cryogenic-temperature specimens. At cryogenic temperature, the microvoids came from the fragmentation of precipitated phases under cyclic tension-compression loading. It reduced the resistance of crack propagation. It could inferred that laves phase when the microvoids distributed on the surface of sample, the microvoids would accelerate the formation of crack at cryogenic temperature.
As for CTOD specimens, the crack propagation path in stable crack propagation region of CTOD specimens were observed by using SEM with SBE mode as shown in Fig 10. 8 sections of every specimen were chosen to collect the laws of crack spreading. It could be assured that the main way of cracks in the region propagate at room temperature was to propagate through the grain boundary as shown in Fig 10(a). However, at low temperature. the commonest way is to propagate along the grain boundary as shown in Fig 10(b). No obvious microvoids were found in CTOD specimens.
It meant that the toughness of grain boundary is higher than that in grain interior at room temperature, but it was just the opposite at low temperature. The γ austenite with fcc crystal structure has not embrittlement temperature and low temperature restrains the deformation of weld metal, so the precipitated phases are the main reasons for fracture toughness decrease at low temperature. Which leads to the fracture toughness of both types of weld is lower at that at low temperature. The more precipitated phases had, the lower toughness had. It is clear that the effect of precipitated number on the fracture at room temperature is greater than that at low temperature.

Facture morphology characteristics
It is recognized that fatigue fracture of LCF specimen is made up of three regions: crack initiation area (CIA), crack propagation area (CPA) and final rupture area (FRA). The fracture morphology of CIA and CPA region were observed by scanning electron microscopy (SEM). It was found that the cracks in specimens tested at room temperature initiated almost from the surface (Fig 11).
The crack initiation area of low temperature joint was from the surface as well, which was because of the incision and stress concentration at the surface Fig 11(c), (Fig 11(d)). Fig 12 shows the SEM of the crack propagation area of specimens under total strain amplitudes. It is found that fatigue striations, caused by repeated plastic blunting-sharpening process [19], were observed in the crack propagation area of all specimens. Second cracking was found only in cryogenic temperature specimens. Beside of fatigue striations, cleavage river pattern and extrusion ridges were found in all specimens, which mean that the crack propagation area of all specimens were ductile-brittle mixed fractures. The distance of striations increased with the total strain amplitudes. It was mostly the same at cryogenic temperature. It was because that the plastic strain amplitude increased with the increase of total strain amplitude.
As shown in Fig 13, the fracture morphology in SCP region of representative CTOD specimens were examined by scanning electron microscopy (SEM). For weld seam specimens, the dimples at room-temperature specimens, were bigger and deeper than that in corresponding low-temperature specimens, as well as fusion area specimen as shown in Fig 13 (a) and Fig 13  (b). It's in accordance with the measured values of CTOD. The low-temperature specimens still showed ductile fracture characteristics to some extent since some dimples were found. It was also found that the dimples in weld seam were obviously shallower than that in fusion zone, and the cleavage characteristic appeared locally, as shown in Fig 13 (c). With the decrement of testing temperature, the cleavage characteristic of fracture surface increased, as shown in Fig 13 (d). The differences of fracture surface was consistent with the CTOD values.  At cryogenic temperature, under the same total strain amplitude, the fatigue life of 9%Ni steel welded joint increased, due to the higher fatigue strength. Most fracture initiation of joints located in fusion area at room temperature, while it occurred in weld seam at cryogenic temperature. 2. The fracture toughness of weld metals was tested using CTOD at room temperature(296 K) and at low temperature(80 K), respectively. Experimental results showed that the weld seam with less precipitates had higher CTOD values no matter the testing temperature. 3. The effect of precipitated phase was the true reason for these behavior. The fatigue cracks propagated in transgranular mode at room temperature, ultimately, and intergranular mode at low temperature in both LCF specimens and CTOD specimens.