Welding plays a key role in all types of fabrication. This article discusses the mechanism of welding different categories of stainless steel with carbon steel, galvanised steel, aluminium, copper and tungsten.
By A. K. Vaish, Ritesh Patel*, B. J. Chauhan, S. D. Kahar, Ankit Bhojani, K.M.Bhaisaheb and Keshva Narayana.* Department of Metallurgical and Materials Engineering, The M.S.University of Baroda, India;
*Larsen and Toubro Ltd., Hazira.
Welding is a fabrication process in which metals are heated, melted and mixed to produce a joint, whereby two or more parts are fused together. When used with filler material, it produces a stronger weld than the base material. The weldability of stainless steel is influenced by its alloying elements.
Versatile stainless steel
Stainless steel comprises at least 10.5% chromium, <1.2% carbon and other alloying elements such as nickel, molybdenum, nitrogen, titanium, niobium, manganese, etc. Chromium forms a stable oxide (Cr2O3) film on the surface, which is continuous, impervious and passive to stop any further reaction between steel and the surrounding atmosphere and protects the steel from further oxidation. A higher chromium content in stainless steel leads to higher resistance to oxidation. It possesses higher strength at room temperature as well as at high temperatures.
Stainless steel can be grouped into five main categories:
- Ferritic stainless steel
Its microstructure is ferrite with a body-centered cubic grain structure. It is non-hardenable by heat treatment and marginally hardenable by cold rolling. - Martensitic stainless steel
It is austenitic at temperatures 950-1000°C, but transforms to martensite on cooling to room temperature. This steel can be quenched in oil or air from 1050°C (when fully austenitic) and then tempered to obtain a yield strength of 550-1860 Mpa. - Austenitic stainless steel
Has an austenite crystalline structure as well as a face-centered cubic structure. This steel is a single-phase FCC material and can be strengthened by cold working and cold solution strengthening. - Duplex stainless steel
Contains low carbon and is a mixture of ferrite (bcc) plus austenite (fcc). This steel has both structures and gets the benefit of both phases. - Precipitation–hardening stainless steel
An iron-nickel-chromium alloy containing one or more precipitation-hardening elements such as aluminium, titanium, copper, niobium or molybdenum. It is a special alloy produced by vacuum melting. Different categories of stainless steels with their chemical compositions are depicted in Figure 1.
Mechanism of welding of stainless steel
It is essential to acquire details of the composition and properties of stainless steel to comprehend the welding mechanism, which depends upon:
- Carbon content
- Alloying elements
- Grain coarsening at high temperature
- Type of filler and its composition
- Pre- or post weld heat treatment
- Hardening effect of heat on the welded joint
As the composition of stainless steel changes from one category to another, the welding mechanism changes accordingly.
Ferritic stainless steel
Ferritic stainless steel is predominantly single-phase and non-hardenable and is readily fusion welded. It undergoes rapid grain growth at high temperatures, leading to brittle, heat-affected zones (HAZ), and it therefore has poor weldability. Usually, ferritics are welded in thin sheets or sections less than 6 mm thick. Filler metals that match or exceed the chromium level of the base alloy should be used. Ferritic stainless steel may crack during welding due to excessive grain coarsening, leading to poor toughness in the heat-affected zone. Caution is required for welding thin sections. With thicker sections, a low heat input can minimise the size of the grain coarsened zone and, therefore, sensitivity to cracking [1]. Ferritic stainless steel is generally welded using the TIG process, offering high quality, versatility and longevity. The low heat input of the TIG process makes it ideal for thin material.
Martensitic stainless steel
There are two types of martensitics: low-carbon (between 0.05 to 0.35 %) high strength and high-carbon (between 0.60 and 1.5 %) high hardness. Martensitics are vulnerable to cool cracking and do not weld easily. Low-carbon martensitic stainless has high strength and good weldability, whereas high-carbon martensitic stainless has low weldability and toughness and increased carbides. This steel becomes hard and brittle upon cooling. Precautions must be taken to avoid cracking in the HAZ, especially in thick section components. High hardness in the HAZ makes this type of steel very prone to hydrogen cracking. When the oint cools to room temperature, the weld is completely transformed to untempered martensite. The risk of cracking increases with the carbon content and is combated using hydrogen-controlled fillers.
For thicker sections and higher carbon materials, pre- and post-weld treatments are carried out to reduce the risk of cracking [2]. The filler metals are selected to match the chromium and carbon content of the martensitic steel.
Austenitic stainless steel
Austenitics have a crystalline structure and a face-centred cubic lattice structure. It has high weldability, ductility, toughness and formability. Austenitic stainless steel is readily welded using any arc welding process, namely TIG, MIG, MMA or SA. Since it is non-hardenable on cooling, it exhibits good toughness, and it does not require pre- or post-weld heat treatment. Normally, fillers with a matching composition to the base material are used. This steel is a single-phase FCC material; it can be strengthened by cold working and cold solution strengthening.
Duplex (austenitic+ ferritic) stainless steel
Duplex is an almost equal mixture of ferrite (bcc) plus austenite (fcc), combining the toughness and weldability of austenite with the strength and resistance to localised corrosion of ferrite. It has higher tensile and yield strengths and better weldability and formability than austenitic and ferritic stainless steels. Duplex stainless steel’s weldability is better than ferritic stainless steel’s and not as good as austenitic stainless steel. Duplex is welded with a relatively high heat input and low interpass temperatures. Welding temperatures should be chosen carefully, as too much heat can compromise the structural integrity of the steel. Weld filler is selected more carefully since filler metal cools much more quickly than the base metal.
Precipitation hardening stainless steel
PH stainless steel is a combination of martensitic and austenitic stainless steel. While its weldability is not on par with austenitic stainless steel, it is still very good. It is welded without preheating treatment but is heated after the weld is complete to preserve its structural integrity. PH stainless is readily welded through standard fusion and resistance methods. Special care is needed during heat treatment to achieve optimum mechanical properties in the weld and the parent material. The matching filler is selected, and heat treatment after welding helps the weld to achieve close similarities to the parent material.
The salient features of the mechanism of welding of different categories of stainless steel are shown in Figure 2.
Welding stainless steel to other metals
Stainless steel components are often welded to components made of other metals. Extreme differences in the melting points of metals make them more difficult to join using standard methods.
Plain carbon steel – Austenitic stainless steels, e.g. 304 or 316, are welded to plain carbon steel using MIG and TIG welding. Filler materials are preferred during MIG welding. Due to differences in electrical conductivity between stainless and plain carbon steel, it
is difficult to reach the correct weld temperature. Resistance welding can be used if carbon steel is preheated, as it is more electrically conducive and does not heat up as fast as stainless steel.
Low carbon plain steel (mild steel) – The carbon content in low carbon plain steel (mild steel) typically ranges from 0.05% to 0.25% by weight. The welding of low carbon steel to stainless steel is not difficult since these two metals have almost similar properties. MIG welding or GMAW is an excellent process to weld stainless steel to low carbon steel. The most important thing is to select the proper wire. The best filler metal for welding stainless steel to low carbon steel is the 309[3]. This filler material has low carbon content and a small amount of ferrite to prevent cracking. Grade 309 has enough chromium and nickel to counter the low carbon steel dilution problem. As a result, the deposited weld metal will have excellent corrosion resistance.
Medium carbon plain steel – Medium carbon steel containing 0.30-0.60% carbon and 0.60-1.65% manganese is stronger than low carbon steel but is more difficult to weld, and is more prone to cracking. Medium carbon steel is welded with stainless steel using
a low-hydrogen welding process or controlled hydrogen fillers.
High carbon plain steel – High carbon steel containing 0.60-1.0% carbon and 0.30-0.90% manganese is extremely hard and strong. It has poor weldability and is difficult to weld without cracking. As the carbon content of steel increases, the steel becomes stronger and harder as well as less ductile. High carbon steel is typically considered “hard to weld” due to the hardening effect of heat at the welded joint. It may readily form the hard and brittle martensite phase as it cools from welding. Because of the high carbon content and the heat treatment usually given to this steel, its basic properties are impaired by arc welding. Therefore, this steel requires thorough preheating and post-heating to avoid this. Austenitic stainless steel, such as grade 304 stainless or grade 316, can be welded to plain carbon steel using MIG and TIG welding. During MIG welding, filler material is preferred [4].
Galvanised steel – Stainless steel has good weldability to galvanised steel. The zinc coating around the area to be joined is removed before welding as molten zinc, if present in the weld fusion zone, can result in embrittlement or reduced corrosion resistance of the finished weld.[5] Aluminium – It is possible to weld aluminium with stainless steel. Aluminium-steel-aluminium joints find applications in aerospace, automotive and shipbuilding to reduce the weight of the structure, thereby enhancing efficiency. There is a huge difference in melting point between aluminium alloy and steel, which makes this a great disadvantage for the process. Stainless steel and aluminium are joined via arc welding; two special techniques have been developed to isolate the metals from each other during the arc welding process. Very brittle intermediate compounds are not formed during welding.
- The first method uses bimetallic transitions, in which aluminum and stainless steel are joined by methods that do not create the compound and allows the joining of the two metals by only welding aluminium to aluminium and stainless steel to stainless steel.
- In the second method, the stainless steel is coated with aluminium. This is sometimes achieved by dip coating (hot dip aluminising) or brazing the aluminium to the surface of the steel. Once coated, the steel member can be arc welded to the aluminium member, if care is taken to prevent the arc from impinging on the steel [6]
Copper – Copper and stainless steel can be welded together, but it is extremely difficult and offers very little structural strength.[7] The melting point of stainless steel is much higher than that of copper. Electron beam welding (EBW) is the preferred welding process, mainly because EBW is a great process for welding copper in general.
Tungsten – Tungsten is an ideal welding material for stainless steel due to its high melting point and strength and is used to easily weld stainless joints. Thoriated tungsten electrodes, containing 1-2% thorium oxide (ThO2), are ideal for high amperage welding of stainless steel. Tungsten is welded in a very pure atmosphere of either inert gas (gas tungsten-arc process) or vacuum (electron beam process) to avoid contamination of the weld by interstitials [8]
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This Featured Story appeared in Stainless Steel World December 2023 magazine. To read many more articles like these on an (almost) monthly basis, subscribe to our magazine (available in print and digital format) – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.
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