The use of duplex stainless steel filler metals to avoid hot cracking in GTAW welding of austenitic stainless steel AISI 316L

Sulfur is an element that is intrinsically and sometimes even deliberately present in stainless steel. It is usually bonded in the form of manganese sulfides, which at low levels can have a significant influence on improving machinability. In this work, solidification cracking in austenitic stainless steels welds was investigated. The solidification mode of stainless steels is of fundamental importance and most austenitic stainless steels are designed to solidify to give primary ferrite and secondary austenite to minimize the occurrence of hot cracks. The primary austenitic solidification mode enables cracks to initiate and propagate more easily. This is further enhanced by sulfur segregation. The primary ferritic mode of solidification, however, inhibits crack initiation and propagation and promotes backfilling. The ability to backfill the cracks also affects the extent of cracking observed in welds. Different filler wires were tested to weld, through GTAW welding process, tubes of type 316L UNS S31603 to forged fittings of type ASTM A182 F316 that presented sulfur and phosphorous contents, respectively, 0.03% and 0.045% wt. Duplex stainless steel filler metals ER 2209 and ER 2594, represented a creative solution to avoid hot cracking observed on those samples welded using austenitic stainless steel filler metals ER 316L and ER 309L. Several complementary techniques of microstructural analysis were used, such as optical emission spectrometry, optical microscopy and scanning electron microscopy with coupled EDS Keywords— Austenitic Stainless Steels; Solidification Mode; Hot Cracking.


Fig.1: Comparative machinability of frequently used stainless steels and their free-machining counterparts. % based on
100% for AISI type 416 free-machining stainless steel [1] However, there is a dark side to these high sulfur additions. Sulfur attacks the good attributes of stainless steels. Corrosion is compromised, interferes with welding and can become an initiation site for cracking to occur, especially when any deformation is performed on the part or when there are thin wall sections. The use of sulfur also found its way into other common stainless grades like 410, 304/304L and 316/316L. The adverse effects of sulfur in these grades are not as pronounced on properties as the free machining grades. Welding, corrosion resistance and ductility are generally not an issue. These small sulfur additions do have a substantial effect on the machinability of the stainless steels, as a 0.005% in weight increase can improve machinability by 30% or more. [1,2] The possible solidification modes in the Fe-Cr-Ni system are:

Austenitic solidification (L  L+  ):
The only solid phase to form is austenite. In austenitic solidification, called solidification mode I, there is no other phase transformation at high temperature. Austenite solidifies as a primary phase in a dendritic or cellular way. As the temperature decreases, ferrite  is formed from the remaining liquid. Solidification occurs through a peritectic reaction (L+). This is called solidification mode II. The duplex stainless steels solidify according to ferritic-austenitic solidification (LL+L+++).  ferrite solidifies as the primary phase in dendritic or cellular fashion. As temperature decreases, austenite is formed by a peritectic (L+) or eutectic (L+) reaction. In the case of a peritectic reaction, the initially formed austenite completely surrounds the ferrite and subsequently grows into ferrite and liquid. Depending on the rate of diffusion through the austenite, the reaction may or may not be complete, and at the end of the solidification ferrite may be involved in austenite. Between the two reactionsperitectic and eutectic -the transition takes place where, during the initial formation of austenite by peritectic reaction, ferritizing elements secrete to the liquid, provoking their enrichment in these elements and consequently the simultaneous formation of ferrite and austenite by means of a eutectic reaction. This is called solidification mode III. [3][4][5][6][7][8][9][10][11][12][13]

IV) Ferritic solidification (LL+):
The only solid phase to form is ferrite. In ferritic solidification, called solidification mode IV, ferrite is The solidifications of austenitic stainless steels can occur according to the first three solidification modes, being therefore possible to obtain a "completely austenitic" matrix according to the Fe-Cr-Ni equilibrium diagram shown in figure 2.   [14] It is observed in figure 3 that the solubility limit of sulfur in δ ferrite is 0.14 % in weight and in γ austenite is 0.05 % in weight. Figure 4 presents the Fe-P equilibrium diagram and respective solubility limits of phosphorus in the allotropic phases of iron, α ferrite and γ austenite.

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[  [14] It is observed in figure 4 that the solubility limit of phosphorus in α ferrite is 2.80 % in weight and in γ austenite is 0.31 % in weight.
The information taken from both Figures 3 and 4, helps to understand why solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in solidification modes I, austenitic solidification, and II, austenitic-ferritic solidification. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as sulfur and phosphorus, and alloy elements such as titanium and niobium. [15] Sulfur is known to be an undesirable impurity in welding of stainless steels due to the formation of low-melting sulfide films along the interdendritic and grain-boundary regions. Sulfur is strongly rejected into the liquid during solidification of austenite, rapidly lowering the melting point of the interdendritic liquid. Thus, the potential for forming low-melting eutectics remains strong even with very low contents of sulfur in austenite (< 0·005 wt.%). On the other hand, δ-ferrite shows higher solubility for elements like sulfur, phosphorus, silicon and niobium. [15] Manganese additions are well-known to decrease cracking in steels that present high content of sulfur by forming higher-melting MnS-γ eutectic in preference to Fe-FeS.
Further, the addition of lanthanum and other rare earth elements has been found highly effective in binding the P and S as stable compounds.
[15] Table 1 presents the most important eutectic reactions involving sulfur and phosphorus during the solidification of commercial stainless steels. with respect to the liquidus surface, which under equilibrium conditions proceeds toward the eutectic/peritectic before solidification is complete. Figure 5 shows the pseudo-binary equilibrium diagram on the vertical section of Fe-Cr-Ni equilibrium diagram at a constant Fe content of 70% in weight. It is commonly used to identify the primary solidifying phases or solidification modes for various compositions of different stainless steels. [3, 4, 15]

[3]
Sulfur is known to be an undesirable impurity in welding of stainless steels due to the formation of low-melting sulfide films along the interdendritic and grain-boundary regions. Sulfur is strongly rejected into the liquid during solidification of austenite, rapidly lowering the melting point of the interdendritic liquid. Thus, the potential for forming low-melting eutectics remains strong even with very low contents of sulfur in austenite (< 0·005 wt.%). On the other hand, δ-ferrite shows higher solubility for elements like sulfur, phosphorus, silicon and niobium. [15] Manganese additions are well-known to decrease cracking in high-S steels by forming higher-melting MnS-γ eutectics in preference to FeS. Further, the addition of lanthanum and other rare earths has been found highly effective in binding the P and S as stable compounds. [15] According to studies by Suutala [16][17][18][19][20][21], the Creq/Nieq ratio is fundamental in determining the solidification mode of austenitic stainless steels. Figure 6 presents the solidification cracking behavior in austenitic stainless steels welds as a function of Creq/Nieq ratio and P+S levels. It is observed in figure 6 that austenitic stainless steels that present P+S wt% below 0.01%, are not susceptible to hot cracking. When the Creq/Nieq ratio is below 1.5, if the total P+S wt % is higher than 0.01%, the austenitic stainless steels welds are very susceptible to hot cracking. If 1.5 < Creq/Nieq < 1.75, the austenitic stainless steels welds are slightly susceptible to hot cracking. Finally, when the Creq/Nieq ratio is higher than 1.75, the austenitic stainless steels welds are not susceptible to hot cracking even for total P+S wt % higher than 0.20.

II. EXPERIMENTAL
Four pairs of tubes of type 316L UNS S31603, and forged fittings of type ASTM A182 F316 (weldolets), from the same heats, were welded with different welding wires through GTAW process but keeping the welding parameters as equal as possible.
The shielding gases used were 99.99% Ar to samples 1 & 2, 98% Ar+2% N2 to samples 3 & 4, and the purge gas used was the same 99.99% Ar to all the samples.
The specimens were removed from the base metal and the joints of the tubes using a cut-off.
Chemical analyzes were carried out in all samples by means of an optical emission spectrometer, according to ASTM E 1086-08. [24] Afterwards, the samples were embedded in hot-cure resin (bakelite). The conventional manual polishing was applied using water slicks (100, 240, 320, 400, 600 and 1000 mesh) in order to standardize the surface finish of the samples. Afterwards, a cloth polishing with 9, 3 and 1 μm diamond abrasive paste was carried out in this sequence.     compositions obtained from table  3, table 4 presents the calculations of PREN, Creq, Nieq, Creq/Nieq ratio and total P+S wt %.

III. RESULTS AND DISCUSSION
The calculation of Creq, Nieq and PREN were done using Equations 1, 2 and 3, respectively. The results shown on tables 3 and 4, confirm that the four filler metals chosen to run the tests, presented PRENs higher than that of the tube UNS S31603. That resulted in chemical compositions of the all weld metals of the samples 1, 2, 3 and 4 that have PRENs above that of the base metal with lower PREN, that in this study is the tube of Type 316L UNS S31603.
The calculation of the Creq/Nieq ratio, and total P+S wt %, showed that both base metals presented Creq/Nieq ratios below 1.5 and the total P+S wt % higher than 0.01%. The same was observed on the all weld metal of sample 2, welded using the filler metal ER 309L. This is an indication that these austenitic stainless steels are very susceptible to hot cracking.

International Journal of Advanced Engineering Research and Science (IJAERS)
[ Although the four all weld metals from samples 1, 2, 3 and 4, showed P+S wt % higher than 0.01%, it is interesting to verify that sample 1 presented Creq/Nieq ratio equal to 1.62 indicating that this joint is slightly susceptible to hot cracking. In the case of samples 3 and 4, welding using duplex and super duplex filler metals, respectively, ER 2209 and 2594, the Creq/Nieq ratios are higher than 1.75, resulting that these dissimilar stainless steels welds, solidify in a ferritic-austenitic (mode III) or ferritic (mode IV) fashions. It is expected that these joints are not susceptible to hot cracking even for total P+S wt % higher than 0.20. Table 5 presents the results of the mechanical properties of the samples 1, 2, 3 and 4. Both samples 3 and 4, welded using duplex and super duplex stainless steels filler metals, respectively, ER 2209 and ER2594, showed higher tensile test results than base metals, being in this way considered approved.
In the other hand, both samples 1 and 2 showed lower tensile test results than base metals. As discussed before, both all weld metals of samples 1 and 2 are prone to solidification cracks.    The analysis of figures 8 and 9, shows that the regions close to the cracks have higher sulfur and phosphorus contents than the regions away from the cracks.
This fact reinforces the theory that micro segregations of sulfur and phosphorus during the solidification of austenitic stainless steels that present Creq/Nieq ratio below 1.75 can generate solidification cracks.
Austenitic stainless steels are, usually, indicated for high temperature applications [27]. However, it is important to emphasize that duplex stainless steels are not recommended for high temperature applications, due to the fact that these stainless steels are prone to the precipitation of deleterious phases, as shown at figure 10.

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[

IV. CONCLUSIONS
When the Creq/Nieq ratio is lower than 1.75, the solidification may be austenitic (mode I) or austenitic-ferritic (mode II). If the total content of phosphorous and sulfur is higher than 0.01%, the all weld metal is susceptible to hot cracking.
Sulfur and phosphorous are strongly rejected into the liquid during solidification of austenite, rapidly lowering the melting point of the interdendritic liquid. On the other hand, δ-ferrite shows higher solubility for elements like sulfur, phosphorus, silicon and niobium.
Due to the ferritic-austenitic solidification (mode III), duplex stainless steel filler metals, demonstrate to be efficient in the welding of austenitic stainless steels that present total content of phosphorous and sulfur higher than 0.048%.