Welding of Stainless Steel

4. Recommended welding consumables for similar metal joints

(1) Recommended welding consumables for martensitic and ferritic stainless steels

Basically, the welding consumables with the chemical composition similar to that of the base metal are selected. (See Table 5.)

309−type welding consumables can also be used for Cr stainless steel. In this case, however, caution is required because there is a fear that thermal fatigue can occur in thermal cycles because the thermal expansion coefficient is different between the base metal and the weld metal.

Table 5 Recommended welding consumables for martensitic and ferritic stainless steels
Steel grade
JIS (AISI)
Recommended welding consumables
SMAW covered electrode MAG welding wire (FCW) TIG welding wire
SUS410 (410) CR−40
NC−39*1
DW−410Cb
DW−309*1
TG−S410
TG−S309*1
SUS410S (410S) CR−40
NC−39*1
DW−410Cb
DW−309*1
TG−S410
TG−S309*1
SUS405 (405) CR−40Cb
NC−39*1
DW−410Cb
DW−309*1
TG−S410Cb
TG−S309*1
SUS430 (430) CR−43
NC−39*1
DW−309*1 TG−S309*1
SUS430LX (-) CR−43Cb
NC−39*1
DW−309*1 TG−S309*1
SUS444 (444) NC−36L
NC−39MoL
DW−316L
DW−309MoL
TG−S316L
TG−S309MoL
(Note)
*1. It is better to avoid using this type of consumables where the weldment is to be used in a thermal−cycle environment or in a Ni−sensitive corrosion environment.

(2) Recommended welding consumables for austenitic stainless steel

Basically, the welding consumables with the chemical composition similar to that of the base metal are selected. (See Table 6.)
When flux−cored wires are used for welding a structure that consists of austenitic stainless steel of SUS304 (AISI 304) or SUS316 (AISI 316) grade for 500℃ or higher temperature services, specific wires for high temperature services should be used.
Low−carbon welding consumables may be used for such stainless steels with ordinary carbon content as SUS304 (AISI 304) and SUS316 (AISI 316). This may not be applicable in a service environment where high temperature strength (such as creep strength) is required.
Low−carbon stainless steels such as SUS304L (AISI 304L) and SUS316L (AISI 316L) contain a maximum carbon of 0.03% while the matching welding consumables contain a maximum carbon of 0.04% in accordance with the respective standards. Therefore, when the same carbon content is required for the weld metal as that of the base metal, extra−low−carbon welding consumables should be used.
Table 6 Recommended welding consumables for austenitic stainless steels
Steel grade
JIS (AISI)
Recommended welding consumables
SMAW covered electrode MAG welding wire (FCW) TIG welding wire
SUS304 (304) NC−38
NC−38H*1
DW−308
DW−308H*1
TG−S308
SUS304L (304L) NC−38L DW−308L    DW−308LH*2
DW−308LP*3    DW−T308L*4
TG−S308L
SUS304LN (304LN) DW−308LN
SUS309S (309S) NC−39 DW−309    DW−310*1
DW−309LP*3    DW−T309L*4
TG−S309
SUS310S (310S) NC−30 DW−310 TG−S310
SUS316 (316) NC−36 DW−316    DW−316LH*1
DW−316LP*3
TG−S316
SUS316L (316L) NC−36L DW−316L    DW−316LH*2
DW−316LP*3    DW−T316L*4
TG−S316L
SUS316LN (316LN) NC−317L DW−317L
DW−317LP*3
TG−S317L
SUS317 (317) NC−317L DW−317L
DW−317LP*3
TG−S317L
SUS317L (317L) NC−317L DW−317L
DW−317LP*3
TG−S317L
SUS321 (321) NC−37
NC−37L
DW−347
DW−347LH*1
TG−S347
SUS347 (347) NC−37
NC−37L
DW−347
DW−347LH*1
TG−S347
SUS329J3L (31803) NC−329M DW−329A
DW−329AP*3
TG−S329M
SUS329J4L (32250) NC−329M DW−329M TG−S329M
SUS630 (S17400) TG−S630
(Note)
*1. For high−temperature specification.
*2. For SR (stress relief annealing) specification.
*3. For all−position welding.
*4. For thin to thick steels due to a wide range of applicable welding currents.

5. Recommended welding consumables for dissimilar metal joints

Welding of two kinds of steel different in chemical composition is called dissimilar metal welding.

In dissimilar metal welding, selection of welding consumables requires sufficient consideration of crack resistance, corrosion resistance, and mechanical properties according to the combination of base metals.

Table 7 shows the commonest welding consumables that are used for dissimilar metal welding.

Basically, such welding consumables should be used that satisfy the mechanical properties of at least one of the base metals of the joint.

Below are supplementary comments to Table 7.

In welding of carbon steel with austenitic stainless steel, 309−type welding consumables with higher Cr and Ni are ordinarily used. This is because, with 308−type welding consumables, Cr and Ni can be diluted by the carbon steel base metal and thus martensitic structure (brittle structure) can be formed in the weld metal.
In welding of carbon steel with austenitic stainless steel, as these two metals are very different in the thermal expansion coefficient, Inconel type welding consumables with high Ni content which has intermediate thermal expansion coefficient should be used where the weldment is subjected to intense thermal cycles in service.
In welding of carbon steel with Cr stainless steel, any of the welding consumables of Cr stainless type, austenitic stainless type and high Ni type can be used in consideration of the following advantages and disadvantages for each type.
Cr stainless type consumables are suitable for an application under intense thermal cycles or Ni−sensitive corrosion environments. However, appropriate preheating and postweld heat treatment are required to prevent delayed cracks.
Austenitic stainless type consumables are good in weldability but cause the thermal stress problem in an environment where the weldment is exposed to intense thermal cycles.
High Ni type consumables are costly and prone to generate hot cracks ; on the other hand, since they endure intense thermal cycles, they are suitable for the weldment that is difficult to be postweld heat treated and is used in an environment of intense thermal cycles.
For welding dissimilar metals, such a welding process that features a big dilution ratio as submerged arc welding is not recommendable.
When the MIG and TIG welding processes are used for welding dissimilar metals, penetration into the carbon steel should be kept as small as possible.
Table 7 Recommended welding consumables for dissimilar metal welding*1*2
Base metal
combination
Welding
process
Austenitic stainless steel
(JIS : US304, 316, 347, etc.)
(AISI : 304, 316, 347, etc.)
Ferritic and martensitic
stainless steel
(JIS : SUS410, 430, etc.)
(AISI : 410, 430, etc.)
Mild steel,
Low alloy steel
Shielded metal
arc welding
NC−39
NI−C70A
NC−39, NI−C70A
CR−43Cb
MIG welding MG−S309LS
MG−S70NCb
MG−S309LS
MG−S70NCb
FCW welding DW−309, DW−N70A DW−309, MG−S430M,
DW−N70A
TIG welding TG−S309
TG−S70NCb
TG−S309
TG−S70NCb
Ferritic and martensitic
stainless steel
(JIS : SUS410, 430, etc.)
(AISI : 410, 430, etc.)
Shielded metal
arc welding
NC−39, NI−C70A
MIG welding MG−S309LS
MG−S70NCb
FCW welding DW−309, DW−N70A
TIG welding TG−S309, TG−S70NCb
Austenitic
stainless steel
Each welding
process
A welding consumable that
matches the lower alloy
(especially Cr and Ni) base metal
(Note)
*1. For dissimilar metals welding, a 309−type welding consumable is most commonly used but is not suitable for intense thermal cycle applications because its thermal expansion coefficient is different from that of the ferritic base metal.
*2. An Inconel type welding consumable can provide metallurgically stable welds, but it is expensive and apt to be sensitive to hot cracking.

6. Preheating and postheating

(1) Similar metal welding

The proper preheating and postheating conditions in the welding of similar chemistry base metals are given in Table 8.

In the welding procedure control, the key point is the heat control. Especially, with ferritic and martensitic stainless steel (also known as Cr stainless steel), the heat control of preheating and postheating largely determines the results of welding.

Table 8 Preheating and postheating conditions for similar metal welding
  Martensitic stainless steel Ferritic stainless steel Austenitic stainless steel
Preheat temperature 200~400℃ 100~200℃ Not required
Postheat temperature 700~760℃ Normally not required
Purposes of preheating Prevent delayed cracking ①
・Prevent HAZ from hardening
・Help remove hydrogen
Prevent delayed cracking ①
・Help remove hydrogen
Preheating is not normally
applied to avoid degradation
of corrosion resistance
Purposes of postheating Prevent delayed cracking ①
・Softening of HAZ
・Removal of hydrogen
・Relief of residual stresses
・Improve mechanical properties
Prevent delayed cracking ①
・Removal of hydrogen
・Relief of residual stresses
・Improve notch toughness
・Improve corrosion resistance and mechanical properties
(Solid solution heat treatment ②)
・Prevention of stress corrosion cracking
(Stress relief annealing ③)
Remarks ・Hardening
・Hot crack ④
・Corrosion resistance of HAZ ⑤
・475℃ brittleness
・Embrittlement by high temperature heating (900℃ or higher)
・Sigma phase embrittlement (600~800℃)
・Hot cracking ④
・Corrosion resistance⑤
(Note)
*1. The postheating mentioned in the table refers to stress relief annealing (SR) except solid solution heat treatment. In general, SR should be started in a furnace immediately after welding is finished before the weldment cools down to room temperature. If this cannot be executed, the weldment should be heated at 300~ 350℃ for 30~60 minutes right after welding is finished to remove hydrogen from the weld metal, which is called immediate postheating.
*2. For austenitic stainless steel welds, postheating is not conducted normally except special cases.

The following is a detailed explanation of ①~⑤ in Table 8.

① Prevent delayed cracking

Delayed cracking occurs after the weldment has cooled down to the ambient temperature. Three main causes are considered to be diffusible hydrogen in the weld metal, hardening of weld metal and HAZ, and joint constraint.

Preheating and stress−relief annealing are effective for prevention of delayed cracks.

Because preheating can reduce the cooling rate of the weldment, it effectively decreases the hardness of the weld metal and HAZ and enhances release of diffusible hydrogen.

Delayed cracking is a problem with Cr stainless steel weldment but not with austenitic stainless steel weldment.

This is because austenitic stainless steel weldment does not harden irrespective of the cooling rate and the dissolved hydrogen does not become diffusible.

Hence, preheating is not required in welding of austenitic stainless steel. On the contrary, preheating may deteriorate corrosion resistance.

② Solid solution heat treatment

Solid solution heat treatment, which is conducted mainly on austenitic stainless steel weldment, is to hold the weldment at 1000~1150℃ for 2 minutes or longer per 1mm of plate thickness, followed by rapid cooling.

During cooling, the weldment should be cooled as quickly as possible in the range of 500~800℃ to avoid the formation of chromium carbide.

When the weldment is held at 1000~1150℃, chromium carbide, sigma phase and ferrite in the weld metal are dissociated in the matrix. By this heat treatment, corrosion resistance, ductility and toughness can be improved and the inner stresses caused by working and welding can be removed.

③ Stress−relief annealing (SR)

The major purposes of SR are prevention of delayed cracking in Cr stainless steel weldments and improvement of mechanical properties.

While, for austenitic stainless steel weldments, prevention of stress corrosion cracking is the main purpose.

However, when corrosion resistance is important or when sigma phase tends to precipitate as in the case of the weld metal of 347−type or 316−type, SR can be harmful in many cases.

Therefore, SR of austenitic stainless steel weldments should be avoided unless it is considered indispensable after examining sufficiently the steel grade, conditions of use, and past experiences of practice.

④ Hot crack

While cracks in Cr stainless steel weldments can occur at ambient temperatures and are called delayed cracks, those cracks of the weld metal of austenitic stainless steel and high Ni alloy can occur immediately after solidification is completed in most cases and are called hot cracks.

In order to prevent the occurrence of hot cracks, welding consumables for general−purpose austenitic stainless steels are so designed that the weld metal contains a few percent of ferritic structure in the austenitic matrix.

For measuring the percentage of ferritic structure in the weld metal, a few types of methods are available : one is to use metallographic structure diagrams ; one is to use measuring instruments ; and the other is to use a microscope.

Different from Cr stainless steel weldments, cracks of austenitic stainless steel weldments cannot be prevented by preheating and postweld heat treatment. To prevent cracks in austenitic stainless steel weldments, it is important to select the proper welding consumable with a low amount of impurities and to use suitable welding procedures.

⑤ Corrosion resistance of HAZ

Austenitic stainless steel is produced so as to possess uniform corrosion resistance normally by means of solid solution heat treatment. But, once it is welded, the corrosion resistance of the HAZ becomes inferior to that of the unaffected zone of the base metal because carbides precipitate in the HAZ.

This carbide precipitation zone is called weld decay, which is formed by heating in the range of 500~800℃ by welding ; as a result, chromium carbides precipitate, thereby decreasing the independent Cr in the matrix that is effective for enhancement of corrosion resistance. Consequently, the corrosion resistance of the HAZ becomes deteriorated.

Though there are cases where weld decay poses no problem in use, some countermeasures are required when the welded structure is used in an environment where intergranular corrosion or stress corrosion cracks tend to occur.

Preventive or improving measures against weld decay are as follows :

(a)
Apply solid solution heat treatment at 1000~1150℃ after welding is finished to decompose chromium carbides.
(b)
Prevent precipitation of chromium carbide by using low−carbon stainless steel of SUS304L (AISI 304L) or SUS316L (AISI 316L), or stabilized stainless steel of SUS321 (AISI 321) or SUS347 (AISI 347).
(c)
Refuse the surface of the HAZ by TIG welding with a small welding heat input.

(2) Dissimilar metal welding

As to the preheating temperature in welding dissimilar metals, the higher preheating temperature between the two base metals is selected ordinarily. Examples of the preheating temperature in welding dissimilar metals are shown in Table 9. Caution is required as a too high preheating temperature in welding dissimilar metals may result in excessive penetration and thus the chemical composition of the weld metal may become improper.

The use of welding consumables for austenitic stainless steel enables to reduce the preheating temperature for prevention of delayed cracks. But, the use of lower preheat temperatures can reduce the preventive effect against hardening of HAZ.

Examples of the postweld heat treatment (PWHT) temperatures in dissimilar metal welding are shown in Table 10.

As the PWHT of the weld joint of dissimilar metals affect both base metals and the weld metal in various ways, in−depth consideration is required as to PWHT conditions or even whether it is really necessary or not.

If an intermediate temperature or a higher temperature is selected for PWHT of a dissimilar metal joint in comparison with the proper PWHT temperature for each base metal, it may exceed the transformation temperature of the base metal whose proper PWHT temperature is lower (ordinarily the base metal with less alloying elements) and, as a result, the properties of the base metal may change entirely. Therefore PWHT temperature should be examined sufficiently beforehand.

With a combination of ferritic and austenitic steels as in the weld joint of mild steel and austenitic stainless steel, it is a common practice to select a lower PWHT temperature in the recommended temperature range for the ferritic steel.

The reason why a lower temperature is selected is to minimize the carbon migration at the weld interface.

Also, it should be kept in mind that these PWHT temperatures are in the range where austenitic stainless steel precipitates carbides and sigma phases.

Table 9 Preheat and interpass temperatures for dissimilar metal welding
Stainless steel Austenitic stainless steel
(JIS : SUS304, 304L, 316, 316L, 347, 321, etc.)
(AISI : 304, 304L, 316, 316L, 347, 321, etc.)
Martensitic stainless steel
(JIS : SUS410, etc.)
(AISI : 410, etc.)
Ferritic stainless steel
(JIS : SUS430, 405, etc.)
(AISI : 430, 105, etc.)
Mild steel, Low alloy steel
Mild steel 200~400℃ 100~200℃
0.5%Mo steel 100~200℃ 200~400℃ 100~200℃
1.25%Cr−0.5%Mo steel 100~200℃ 200~400℃ 100~200℃
2.25%Cr−1%Mo steel 100~200℃ 200~400℃ 200~350℃
Table 10 Postweld heat treatment (PWHT) temperatures for dissimilar metal welding
Stainless steel Austenitic stainless steel
(JIS : SUS304, 304L, 316,
316L, 347, 321, etc.)
(AISI : 304, 304L, 316, 316L, 347, 321, etc.)*2
Martensitic stainless steel
(JIS : SUS410, etc.)
(AISI : 410, etc.)*1
Ferritic stainless steel
(JIS : SUS430, 405, etc.)
(AISI : 430, 105, etc.)*1
Mild steel, Low alloy steel
Mild steel (550~600℃) 600~650℃ 600~650℃
0.5%Mo steel (550~600℃) 600~650℃ 600~650℃
1.25%Cr−0.5%Mo steel (550~600℃) 650~720℃ 650~720℃
2.25%Cr−1%Mo steel (550~600℃) 680~750℃ 680~750℃
(Note)
*1. In the dissimilar metal welding of ferritic or martensitic stainless steel to mild/low alloy steel, use the highest temperature in the lower PWHT temperature range between those temperature ranges recommended for individual base metals.
*2. For a dissimilar metal joint, one component of which is austenitic stainless steel, PWHT can degrade the corrosion resistance of the austenitic stainless steel. This is why the necessity of PWHT should thoroughly be examined in advance.

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