The Story of Steel Part 3. The Creation of Weldable Steels

The Story of Steel Part 3. The Creation of Weldable Steels

We have come a long way in our story of steel, beginning with the formation of iron from the stars we see in the night sky, to its deposit on the earth as iron oxide and to the eventual making of steel. 

In Part 2 of our story, we saw that steel can be made in one of several ways.

• By the traditional route of the “integrated steel mill” employing blast furnaces for reduction of iron ore and then conversion of pig iron to steel by using the Basic Oxygen furnace This method produces large emissions and countries are moving away from it for these reasons

• Electric Arc Furnace (EAF). Using scrap steel which is 100% recyclable

• Direct Reduction Iron (DRI) methods for reduction of iron ore and then steel by EAF

• Other ore reducing methods, for example substituting hydrogen for coke in processes being pursued in Europe to produce iron followed by EAF for steel production

However the steel is made, when it is poured it will be to a certain standard, for example this could be to a CSA or ASTM specification. Such a specification will contain some basic requirements such as chemistry and mechanical strength.

With something in the order of over 3,500 differing grades and approximately two billion metric tons/annum produced, steel is by far the most commonly used metal in the world. It is also one of the most sustainable construction materials. It is the world’s most recycled metal and each year, more steel by weight is recycled in North America than paper, plastic, aluminum and glass combined. When we view all this, we can say, without any hesitation, that steel is truly a wonder material.

Since we have shown in the previous article that we need to reduce the carbon content from the pig iron during the making of the steel we now know that the final carbon content is of great importance. 

We generally classify steels as those alloys of iron that contain less than about 1.7% carbon and anything above this 1.7%, we classify as a cast iron. However, steels with high carbon contents are not weldable in the traditional sense. Table 1 illustrates some steels with increasing carbon contents and their applications.

Table 1 Steels with Increasing Carbon Contents and their Typical Applications.

Weldable steels, the ones you are involved with on a daily basis, typically contain relatively low levels of carbon. Carbon steels with 0.15-0.30% carbon are readily weldable while those with less than 0.2% carbon are considered ideal. As the carbon content increases then the so called “weldability” of the steel decreases. 

Weldability is an important property that we, in the fabrication business, deal with every day. The American Welding Society (AWS) defines weldability as “the relative ease with which a material may be welded to meet an applicable standard”

Let’s look at the chemical elements that make up a simple carbon steel. Iron, is the basic requirement and the other constituents are added to provide certain properties. The following four elements can be listed with an additional two elements that are regarded as impurities, giving six in total.

• Iron (Fe)Is the base metal in the alloy of steel.  It combines with other alloying elements to form all types of steel. Iron in its pure form is relatively soft and weak.

• Carbon (C) is the most important element in the majority of steels, significantly affecting both hardness and strength. The strength increases but the weldability and the ductility decrease with increases in carbon content.  Relative weldability of steels with carbon and/or CE content are given in Table 2. Carbon Equivalent(CE) is an indicator that can help predict the weldability of the base metal and factors in other elements which are additive to the effect of carbon alone.

• Manganese (Mn) contributes to both strength and hardness.

• Silicon (Si). This element is a principal deoxidiser in steel.

• Sulphur (S) and Phosphorous (P) are present from the ore. These are considered to be impurities and are normally controlled to low levels as they increase cracking susceptibility in welds.

Table 2 Relative Weldability of Steels with Carbon and Carbon Equivalent Content

On top of this basic steel, there are other elements that we can add in relatively small amounts, to change or improve the steels properties. This is the territory taken by the so called “alloy” or “microalloyed steels”. We can consider the addition of these elements in a similar vein to adding more ingredients to an omelette in order to improve its taste. In our case we are adding elements to improve certain defined properties of our steel. Some of the most important of these are:

• Chromium (Cr) increases the ability of the steel to harden (defined as hardenability) and improves its wear and abrasion resistance

• Nickel (Ni) increases hardenability, tensile strength and toughness (crack resistance) of a steel.

• Molybdenum (Mo) added to steels to improve strength and hardness.

• Niobium (Nb) helps control the grain size in the steel and thus increase the toughness, strength, and formability of the steel.

• Zirconium (Zr) additions can control grain size, amongst other properties in microalloyed steels

In addition to this, we can go much further and add larger amounts of certain elements to produce, for example chrome/moly heat resistant steels and stainless steels. Stainless steels have certain amounts of chromium, nickel and/or molybdenum added to give them “stainless” and other advanced properties.

So, as it can be seen, there are many elements, some of which not included in the above, that are available to be added to the basic iron and which ultimately defines a steel with certain properties. 

When we come to join metals by welding, the completed joint, in most cases, must have a chemistry that closely matches the base metal and provide similar mechanical properties to the base steel that will be joined.

In Part 4 of the “Story of Steel” we will look at some steel specifications and how we weld them, taking into account the differences of each. We will look at a basic alloy steel, a low alloy steel with unique properties and a stainless steel. 

These alloys are all very different, demanding differing welding approaches. However, they are all steels that began their existence in the very same way, as iron formed in an exploding star known as a “red giant” that “blew up real good”........ way out there in outer space!