Welding of Stainless Steel

1. Features of stainless steel

By adding chromium (Cr) to iron (Fe), iron becomes resistant to corrosion in the atmosphere. When the Cr content increases to 11～12% or more, the corrosion resistance of steel becomes remarkably high.

Hence, the steel with such a high amount of Cr is given a name of stainless steel, where “stainless” means to be free from stain of rust.

The reason why stainless steel has good corrosion resistance is that Cr in it is oxidized in the atmosphere and forms a protective film named “passive film” on its surface.

Depending on the environmental conditions in which stainless steel is intended to use, Cr content is increased and Ni and other elements are also added to the steel.

However, since its corrosion resistance is provided principally with Cr, Cr is an essential element for stainless steel. The JIS standard defines stainless steel as “the alloyed steel containing Cr or Cr and Ni for improving corrosion resistance, generally containing about 10.5% or more of Cr.” Similarly, the AWS Welding Handbook (Vol. 4) defines stainless steels as “alloy steels with a nominal Cr content of at least 11%, with or without other alloy additions.”

Stainless steel is highly heat resistant as well as corrosion resistant and thus its use is versatile, from household products to chemical equipment, ships, rolling stock, food processing machines, architectural materials and nuclear power equipment, as such stainless steel is important for our industries.

2. Various types of stainless steel

Stainless steel can be roughly divided into Cr stainless steel and Cr−Ni stainless steel.

These two grades can further be classified based on their metallographic structure as shown in Fig. 1. Cr stainless steel can be divided into martensitic stainless steel and ferritic stainless steel, and Cr−Ni stainless steel can be divided into austenitic stainless steel, austenite−ferrite stainless steel (duplex stainless steel) and precipitation−hardening stainless steel.

(1) Martensitic stainless steel

A typical grade of martensitic stainless steel as per the JIS standard is SUS410 (AISI 410)(See Table 1.).

It contains 13%Cr and its metallographic structure is martensitic at room temperature that is hard and brittle.

Though good mechanical properties can be obtained with this grade of steel by appropriate heat treatment (tempering), it is considered to be inferior to other grades of stainless steel in corrosion resistance because its Cr content is low.

Uses of martensitic stainless steel are for turbine blades, valves and shafts that require high strength, and abrasion and heat resistance.

Table 1 Chemical requirements for martensitic stainless steels
(extracted from JIS G 4305−1999 and supplemented with AISI)^*1 （％）

Steel grade JIS (AISI)	C	Si	Mn	P	S	Cr
SUS410 (410)	0.15 max.	1.00 max.	1.00 max.	0.040 max.	0.030 max.	11.50~13.50
SUS410S (410S)	0.08 max.	1.00 max.	1.00 max.	0.040 max.	0.030 max.	11.50~13.50
(Note) *1. For the AISI requirements, refer to the relevant specification.

(2) Ferritic stainless steel

Table 2 shows typical grades of ferritic stainless steel.

It contains Cr of about 18% and has a metallographic structure of ferrite that is soft and good in machinability. But it gives metallurgical problems when it is heated at a high temperature.

As compared with martensitic stainless steel, its corrosion resistance is better and even resistant to nitric acid (HNO3) because its Cr content is higher.

Applications of ferritic stainless steel are found widely in the interior and exterior of architectures, kitchen appliances, automobiles, and home electric appliances.

Table 2 Chemical requirements of ferritic stainless steels
(extracted from JIS G 4305 and supplemented with AISI)^*1 （％）

Steel grade JIS (AISI)	C	Si	Mn	P	S	Cr	Mo	N	Others
SUS405 (405)	0.08 max.	1.00 max.	1.00 max.	0.040 max.	0.030 max.	11.50~14.50	－	－	Al : 0.10~0.30
SUS430 (430)	0.12 max.	0.75 max.	1.00 max.	0.040 max.	0.030 max.	16.00~18.00	－	－	－
SUS430LX (－)	0.030 max.	0.75 max.	1.00 max.	0.040 max.	0.030 max.	16.00~19.00	－	－	Ti or Nb : 0.10~1.00
SUS444 (444)	0.025 max.	1.00 max.	1.00 max.	0.040 max.	0.030 max.	17.00~20.00	1.75~2.50	0.025 max.	Ti, Nb, Zr or their total 8× (C%+N%)~0.80
(Note) *1. For the AISI requirements, refer to the relevant specification.

(3) Austenitic stainless steel

Table 3 shows typical grades of austenitic stainless steel.

The commonest grade of austenitic stainless steel is SUS304 or AISI 304 (18%Cr−8%Ni). SUS316 or AISI 316 (18%Cr−12%Ni−2%Mo) offers better corrosion resistance, which is also used widely.

As austenitic stainless steel offers good corrosion resistance, workability, mechanical properties and weldability, it is widely used for fabrication of storage tanks, heat exchangers, wastewater treatment facilities, kitchen utensils, bath tubs, sinks, and so on.

Table 3 Chemical requirements for austenitic stainless steels
(extracted from JIS G 4305−1999 and supplemented with AISI)^*1 （％）

Steel grade JIS (AISI)	C	Si	Mn	P	S	Ni	Cr	Mo	Cu	N	Others
SUS304 (304)	0.08 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	8.00~10.50	18.00~20.00	－	－	－	－
SUS304L (304L)	0.030 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	9.00~13.00	18.00~20.00	－	－	－	－
SUS304LN (304LN)	0.030 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	8.50~11.50	17.00~19.00	－	－	0.12~0.22	－
SUS309S (309S)	0.08 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	12.00~15.00	22.00~24.00	－	－	－	－
SUS310S (310S)	0.08 max.	1.50 max.	2.00 max.	0.045 max.	0.030 max.	19.00~22.00	24.00~26.00	－	－	－	－
SUS316 (316)	0.08 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	10.00~14.00	16.00~18.00	2.00~3.00	－	－	－
SUS316L (316L)	0.030 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	12.00~15.00	16.00~18.00	2.00~3.00	－	－	－
SUS316LN (316LN)	0.030 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	10.50~14.50	16.50~18.50	2.00~3.00	－	0.12~0.22	－
SUS317 (317)	0.08 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	11.00~15.00	18.00~20.00	3.00~4.00	－	－	－
SUS317L (317L)	0.030 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	11.00~15.00	18.00~20.00	3.00~4.00	－	－	－
SUS321 (321)	0.08 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	9.00~13.00	17.00~19.00	－	－	－	Ti : 5×C% min.
SUS347 (347)	0.08 max.	1.00 max.	2.00 max.	0.045 max.	0.030 max.	9.00~13.00	17.00~19.00	－	－	－	Nb : 10×C% min.
SUS329 J3L*2 (31803)	0.030 max.	1.00 max.	2.00 max.	0.040 max.	0.030 max.	4.50~6.50	21.00~24.00	2.50~3.50	－	0.08~0.20	－
SUS329 J4L*2 (32250)	0.030 max.	1.00 max.	1.50 max.	0.040 max.	0.030 max.	5.50~7.50	24.00~26.00	2.50~3.50	－	0.08~0.30	－
SUS630*3 (S17400)	0.07 max.	1.00 max.	1.00 max.	0.040 max.	0.030 max.	3.00~5.00	15.00~17.50	－	3.00~5.00	－	Nb : 0.15~0.45
(Note) *1. For the AISI requirements, refer to the relevant specification. *2. Austenite−ferrite stainless steel (Duplex stainless steel) *3. Precipitation−hardening stainless steel

3. Physical properties of stainless steel

Table 4 shows a comparison of physical properties between stainless steel and carbon steel.

Caution is required in welding of stainless steels as there are big differences in physical properties between stainless steel and carbon steel, which affects weldability directly or indirectly.

For instance, though the thermal expansion coefficient of martensitic and ferritic stainless steel is almost the same as that of carbon steel, that of austenitic stainless steel is 1.5 times as much as that of carbon steel. This indicates that deformation and strain becomes considerably big in welding of austenitic stainless steel than in welding of carbon steel.

Further, if a weld joint that consists of austenitic stainless steel and carbon steel is subjected to thermal cycles, there arise thermal stresses due to difference of the thermal expansion coefficient between the two materials. So, it is a problem to use a weld joint of dissimilar metals including austenitic stainless steel in an environment where temperature changes cyclically.

Still further, as electric resistance of stainless steel is much higher than that of carbon steel, electrode burn tends to occur with stainless steel covered electrodes in shielded metal arc welding. Therefore, the proper welding currents are lower than those for carbon steel electrodes.

Martensitic and ferritic stainless steels are ferromagnetic while austenitic stainless steel is normally nonmagnetic.

However, there are many cases in which austenitic stainless steel weld metal is designed to contain some ferritic structure ; in such cases, it possesses some extent of magnetism.

Existence or non−existence of magnetism is useful for rough judgment of steel grade in relation to welding procedure. For instance, preheating is not applied to non−magnetic stainless steel but preheating is effective to magnetic stainless steel in many cases.

Table 4 Comparison of physical properties between carbon steel and stainless steels

	Carbon steel	Martensitic stainless steel	Ferritic stainless steel	Austenitic stainless steel
Thermal conductivity 10−2Cal/cm/sec℃	Approx.11	Approx.6	Approx.6	Approx.4
Thermal expansion coefficient 10−6/℃	Approx.11	Approx.11	Approx.11	Approx.17
Electric resistance μΩcm	15	57	60	72
Magnetism	Yes	Yes	Yes	No

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