Blog / Stainless steel

Filter

24 January 2020 | Stainless steel

STAINLESS STEEL AND THEIR WELDABILITY

General

Stainless steels are steels alloyed with chromium to improve their corrosion resistance in different environments. Chromium in the stainless steel oxides easily but the thin oxide layer that forms is corrosion resistant, durable and stable. It forms a barrier that prevents corrosive products from oxidizing the alloy (see stainless steel 1 diagram). To be considered stainless, a steel must contain a minimum of 10% chromium.

AISI stainless steel classification method

SERIES	CATEGORY	MAIN ALLOY ELEMENTS	TEMPERED	MAGNETIC
2XX	Austenitic	Cr-Ni-Mn	No	No
3XX	Austenitic	Cr-Ni	No	No
4XX	Ferritic	Cr	No	Yes
4XX	Martensitic	Cr	Yes	Yes
5XX	Martensitic	Cr-Mo	Yes	Yes
6XX	Precipitation hardened	Cr-Ni	Some	Some

Properties and typical uses of each series

Austenitic stainless steels (2XX and 3XX)

The presence of elements that promote austenite formation, such as nickel, manganese or nitrogen, produces austenitic stainless steels. These steels have a total chromium plus nickel content of 23% minimum. They provide a very high work hardening rate when mechanically deformed making it possible to obtain a wide range of mechanical properties through cold working alone. They are non magnetic but can become slightly so when cold worked. Their machinability diminishes rapidly with increased cold working unless they contain sulfur or selenium. These stainless steels provide the best tensile strength at high temperatures and resistance to corrosion and spalling. They are used in the aeronautic, chemical and pulp and paper industries, in heat-treatment equipment, and for making creep-resistant parts, etc.

Common austenitic stainless steels :

Ferritic stainless steels (4XX)

Common ferritic stainless steels :

Martensitic stainless steel (4XX and 5XX)

Precipitation-hardened stainless steels (6XX)

Common martensitic stainless steels :

Physical properties of stainless steels

Heat capacity and thermal conductivity

Coefficient of thermal expansion

Influence of the various alloy elements

Element	Influence	Associated risks
Chromium	– Increases resistance to corrosion and oxidation – Promotes ferrite formation	NONE
Nickel	– Increases high temperature properties – Increases corrosion resistance – Promotes austenite formation	NONE
Manganese	– Desulfurizer – Promotes austenite formation at low temperatures – Promotes ferrite formation at high temperatures	NONE
Molybdenum	– Increases hot mechanical strength – Increases resistance to corrosion in reducing environments – Promotes ferrite formation	NONE
Copper	– Increases resistance to some corrosive agents – Improves stress corrosion resistance – Promotes precipitation hardening – Promotes austenite formation	NONE
Carbon	– Promotes austenite formation – Increases hot mechanical strength	– Reacts with chromium to form a chromium carbide (Cr₂₃C₆) and causes intergranular corrosion
Silicon	– Increases resistance to oxidation – Deoxidizing agent – Promotes ferrite formation	– Promotes hot cracking in ferritic stainless steels
Niobium	– Combines with carbon to reduce tendency to intergranular corrosion risk – Refines the grain – Promotes ferrite formation	– Localized corrosion (crevice) – Hot-cracking
Titanium	– Combines with carbon to reduce tendency to reduce tendency to intergranular corrosion risk – Promotes ferrite formation – Increases corrosion resistance	– Localized corrosion (crevice)
Aluminium	– Increases oxidation resistance – Promotes precipitation hardening – Promotes ferrite formation	NONE
Nitrogen	– Thirty times as effective as nickel in forming austenite – Increases yield strength	– Porosity formation – Hot-cracking
Phosphorus Sulfur Selenium	– Improves machinability	– Reduces weldability by increasing the risk of hot-cracking

Weldability of stainless steels

Preparation

Weldability of austenitic stainless steels

However, increasing the temperature during welding causes chromium carbide precipitation. When heated to between 750 and 1560°F (400 and 850°C), any carbon in excess of 0.02% reacts with chromium to form chromium carbide (Cr₂₃C₆)in the grain boundary. In forming these carbides, six atoms of carbon bond with 23 atoms of chromium; therefore a few carbon atoms can tie up many chromium atoms which are then unavailable to protect the alloy. Chromium impoverishment in the grain boundary adversely affects corrosion resistance and increases the likehood of intergranular corrosion (see stainless steel 2 diagram).

There are three ways to control chromium carbide precipitation :

Stabilize the carbon in the steel by adding titanium or niobium. Carbon has a greater affinity for these elements than for chromium and thus forms titanium and niobium carbides instead of chromium carbides.
Use low carbon stainless steel with less than 0.03% carbon for the base metal and solid wires or 0.04% carbon for other filler metals.
Quench anneal the assembly after welding by heating it to about 2000°F (1100°C), depending on the alloy.

If the nitrogen analysis is not available, use 0.06% nitrogen for GTAW and SMAW, and 0.08% nitrogen for GMAW.

Weldability of ferritic stainless steels

use the lowest possible preheat temperature;
keep the interpass temperature to a minimum;
reduce the current to the minimum recommended for welding;
use the smallest diameter filler metal that is prectical;
make several small passes instead of one wide pass.

Weldability of martensitic stainless steels

Table stainless steel 1 : Preheating of martensitic stainless steels

% Carbon	Preheat Temperature °F (°C)	Post-weld Heat Treatment
‹0.10	minimum 60°F (15°C)	NONE
0.10 – 0.20	400 – 500°F (200 – 260°C)	SLOW COOLING
0.20 – 0.50	500 – 550°F (260 – 290°C)	REQUIRED
›0.50	500 – 600°F (290 – 315°C)	REQUIRED

Weldability of precipitation-hardened stainless steels

Filler metals

Ferrite index calculation method (Ferrite Number)

1- Calculate the nickel equivalent and chromium equivalent for each base metal (if they are different) and for the filler metal using the following formulas :

Nickel equivalent = %Ni + (30 x %C) + (30 x %N) + (0.5 x %Mn)

Chromium equivalent = %Cr + %Mo + (1.5 x %Si) + (0.5 x %Nb)

If the nitrogen analysis is unavailable, use 0.06% nitrogen for GTAW and SMAW or 0.08% nitrogen for GMAW ;

2- Add the nickel equivalents obtained after multiplying by their respective dilution rates, then do the same with the chromium equivalent ;

3- Locate both results on the Delong Diagram in accordance with the chromium and nickel equivalent ;

4- Read the resulting ferrite index and ferrite per cent using the ”Ferrite Index” diagonal lines.

Example

1- Types of material: 304 steel welded using a Sodel 309 electrode

Process : SMAW

Dilution ratios : 70% = Sodel 309 filler metal

30% = 304 base metal

Composition : 304 = 0.06%C ; 1.5% Mn ; 0.8%Si ; 19%Cr ; 10% Ni

309 = 0.04%C ; 1.8% Mn ; 0.5%Si ; 23%Cr ; 13% Ni

Ni equiv. 304 = %Ni + (30 x %C) + (30 x %N) + (0.5 x %Mn)

Ni equiv. 304 = 10 + (30 x 0.06) + (30 x 0.06) + (0.5 x 1.5)

Ni equiv. 304 = 14.35

Cr equiv. 304 = %Cr + %Mo + (1.5 x %Si) + (0.5 x %Nb)

Cr equiv. 304 = 19 + 0 + (1.5 x 0.8) + (0.5 x 0)

Cr equiv. 304 = 20.2

Ni equiv. 309 = 13 + (30 x 0.04) + (30 x 0.06) + (0.5 x 1.8)

Ni equiv. 309 = 16.9

Cr equiv. 309 = 23 + 0 + (1.5 x 0.5) + (0.5 x 0)

Cr equiv. 304 = 23.75

2- Total Ni equiv. = (0.3 x 14.35) + (0.7 x 16.9) = 16.14

Total Cr equiv. = (0.3 x 20.2) + (0.7 x 23.75) = 22.69

4- Ferrite index = 8 FN Ferrite percent = 7.6%

PRACTICAL TIPS FOR WELDING STAINLESS STEEL

1- For a quality weld, you must remove the oxide film through one of the following methods :

use a stainless steel brush or an abrasive disc that has never been used with carbon steel ;
use glass or sand blast ;
machine using a non-chloride cutting fluid ;
strip chemically with a 10 to 20% solution of nitric acid.

2- Surface contaminants such as cutting oil, grease and wax must be removed using a solvent such as acetone.

3- Watch out for porosity with GMAW in particular; because they do not have any deoxydizer like calcium or sodium fluoride as contained in the SMAW coated electrode.

4- Remember that the corrosion resistance of stainless steels decreases as their carbon content increases.

5- Stainless steel electrode weld currents are 10 to 15% less than those for carbon steel because of the higher electrical resistance of stainless steel.

6- Prevent hot-cracking by maintaining a ferrite index greater than 3 FN.

7- To minimize cold-cracking in martensitic steels, use basic electrodes (Sodel XXX-15).

8- Always remove the slag from the bead after welding; it can cause the weld bead to corrode.

Back to list