The Story of Steel Part 4 “Weldability of Specific Steel Alloys”

In this article, Part 4 of our “Story of Steel”, we will take a look at some steel specifications and how we weld them, while taking into account the differences of each. We will look at the following three steels:

1. a basic low alloy steel, 

2. a micro alloy steel with special characteristics and,

3. an austenitic stainless steel. 

These are all steels, but they are all very different and thus they will demand differing welding approaches and solutions. However, you may recall that at the beginning of our journey, they all began life as iron which formed in an exploding star known as a “red giant” and ended up on the Earth as iron oxide.

The initial two steels we will look are both covered by the Canadian Standards Association CSA G40.21 Specification:

Steel # 1. A G40.21 350W Type “Weldable Steel”

This steel is defined as ferritic, a metallurgical term, meeting specified strength requirements and is suitable for general welded construction. The 350 refers to the minimum specified yield strength of 350 Mpa.(Mega pascals).

Chemically, this  steel consists of the basic element’s Iron, Carbon -0.23% max, Manganese and Silicon with controlled amounts of phosphorus and sulphur. The steel also includes a small amount of  so called “grain refining” elements such as niobium, vanadium and aluminum controlled to a maximum of 0.15%. 

This 350W material can be used for general welded construction including such items as buildings and compression members of bridges. This steel is readily weldable and can be joined by any of the arc welding processes with electrode strengths of 490Mpa (70Ksi) i.e., typically the E4918 (E7018) SMAW electrodes or the FCAW wire type E49XT-X (E70XT-X).

Weldability becomes an issue as the thickness increases meaning pre-heats have to be applied after a certain critical thickness. Again, for example, with E4918 low hydrogen electrodes, the minimum preheat for a thickness of 40mm becomes 65C (150F). Preheat slows the cooling rate of the weld and allows any hydrogen to have more time to diffuse from the weld zone. This reduces the probability of HICC, (Hydrogen Induced Cold Cracking) and thus, preheat is a very important parameter when welding these steels. 

Preheat guidelines for many steels are given in Table 5.3 of CSA W 59.

Steel #2. A G40.21 350 Type AT “Atmospheric Corrosion-Resistant Weldable Steel”

This is also a ferritic steel which, on top of the basic chemistry of Fe/C/Mn/P/S and Silicon, has small amounts of corrosion retarding elements and a similar %age of grain refiners as the 350W. It also and has the same specified strength level.

These steels, which may be described as weathering steels, are alloys which were developed to form a stable rust-like appearance if exposed to the weather and thus, negate the need for painting and costly rust-prevention maintenance over the years. In simple terms the steel is allowed to rust and that rust forms a protective layer that slows the rate of future corrosion. The surface colour can vary but is normally a rust brown colour.

The corrosion-retarding effect of the protective layer is produced by the concentration of alloying elements. in it. The steel contains the same elements as the 350W low alloy steel but adds small amounts of Chromium (Cr), Nickel (Ni) and Copper (Cu) to produce the protective corrosion layer.

Applications include primary tension members in bridges and similar elements. Figure 1, showing the rust brown finish, is an illustrative example used in a road bridge that straddles highway 30, southwest of Montreal.

If this steel is made at the mill to provide better corrosion protection, what happens when we weld it. There are several approaches which will depend on the end product required, for example:

• If corrosion resistance and colouring characteristics similar to those of the base metal are required then, the electrode, electrode-flux combination to be used are defined in in standards such as CSA W59. For example, alloy electrodes are specified with choices such as E 5518-C3 (E8018-C3) with SMAW and which contains additions of Cr, Mo and Nickel.

There are other rules dependent on design but, its not our purpose to delve deeply into this. What we see here is that a steel, defined as 350AT with small alloy additions, demands a completely different approach to welding than the 350W with the same strength level. So, while steels may look the same, they can be eminently different and thus must be welded differently.

Figure 1. Example of Weathering Steel Type in Bridge Box Girders.

Steel #3. An AISI 316L Austenitic Stainless

Within the so called 300 series of stainless steels, many have high amounts of Chromium and Nickel and are often known by the popular term 18-8 stainless steels.  The 316L has additional molybdenum, which improves the strength at high temperatures and the corrosion resistance under some conditions. The main alloying elements in this steel, additive to C/Fe are as follows:

• Chromium up to 18.5%. This makes the steel stainless and generally more resistant to corrosive attack.

• Nickel up to 13% This helps produce a non-magnetic austenitic structure giving high temperature strength and resistance to oxidation and corrosion. Again, austenitic is a metallurgical term that will be explained later in our “Story of Steel”

• Molybdenum up to 2.5% is added to improve corrosion resistance. 

Welding the 316L is fairly straightforward with matching consumables readily available. However, there are significant differences in relation to the two steels we have already discussed.  These steels do not suffer from HICC, being austenitic, but we need to be aware of several other things when we join them. The first two can occur during welding whilst the last two are service related but it is necessary to be aware of them. 

Oxidation: Chromium has a great affinity for oxygen, and rapidly oxidizes when heated. This is why, on one side welding of pipe, we use an argon backing gas to prevent oxidation of the weld root. Figure 2 is illustrative.

Figure 2 Illustrative of “with and without back side shielding gas”

Weld Metal Solidification Cracking: Stainless steels like 316L may be prone to weld metal solidification cracking. Methods to avoid them have been developed by using designed welding procedures and electrode composition.

Sensitization: When 316 steel is heated in the temperature range 500°C to 850°C as it would in welding, the HAZ is sensitized by the formation of chromium carbides which can cause a problem in service. The low carbon form 316L does not sensitize and this is the main reason the L versions were developed. To improve weldability.

Stress Corrosion Cracking: Stress corrosion cracking in service can occur as the result of high tensile residual stresses from welding in a particular service environment. This can be alleviated by annealing or stress relief heat treatments to reduce residual stresses. 

So, in discussing the three steels above we have shown there are three very different ways in which you must approach the welding in order to produce an acceptable product. These approaches will be designed by welding engineers and technologists and followed by welding practitioners.

Up to now, our conversation on steel has been macro in nature, that is all the things we have discussed can be seen with the naked eye. However, in this note we have touched on two metallurgical words Ferrite and Austenite. We now need to flip to the micro level to explain these different constituents of our steels. These constituents that will be present in the steel as you weld it and, as your completed joint cools to room temperature.

We will do this in the following articles on our journey along the trail of this wonder material we know as “Steel”