There is a myriad of reasons you might, or might not, like your welds or metal to display a variety of colours.
You may be a sculptor creating a new piece of welded steel artwork, the colours are unlike anything else. Welding in itself is an art. The various colours of both welds and adjacent metals do not just look great, the colours indicate a lot about the weld. They combine to enhance the finished object. An example is when TIG welding stainless steel pipe for the pharmaceutical industry, where any weld colours above gold or straw are not desirable.
Either way, though we have more reliable methods of checking for the ideal temperature, being knowledgeable about colours in welds is important.
Forging, Hardening and Tempering Steel
From the early days of steel production, colour guidance has played a large part in the manufacture of steel and its products. When higher carbon steels are heated, their properties change. This change can make the part useable. If we used an axe or chisel that is too hard, it will shatter when used. In the early days and still in the industry, the colour of the metal is a reliable indicator of tempering (softening) of the material.
Likewise, with welding, the increasing temperature can change the properties of the weld and the material being welded. Blacksmiths will gauge whether a piece of steel is at the right temperature to be worked on by its colour when heated. Forging is carried out when the temperature of the metal is between 750C (red) and 1090C (bright yellow).
Once the forging process is complete, the piece may be hardened by quenching. This is followed by a tempering operation to increase mechanical properties much the same as an axe or chisel. Usually, water or oil is used as a quenching media. Once hardened, the steel can become brittle and liable to shatter in certain applications. To remove some of this brittleness the piece needs tempering. It is reheated to between 230C (tan colour) and 425C (grey colour) depending on the material and allowed to cool naturally.
As an apprentice, you are taught that when resharpening cold chisels and centre punches, do not let the item change colour, keep it cool. Let it go blue and it softens it to the point that it is next to useless. If the process isn’t done correctly, it will go blunt the first time you use it.
Welding Stainless Steel – Colours and Oxidation
There are a myriad of weld colours, or heat-tints, that appear in the weld bead and immediate heat-affected area of stainless steel. These colours can be caused by a number of different factors. Oxygen (ppm) from the atmosphere contaminating the inert shielding gas reacting with the surface of the steel. Different metal compositions can also have an effect. Titanium is particularly susceptible to atmospheric contamination. The process is known as oxidation and, it prevents the natural regeneration of the passive layer of chromium oxide which is essential to austentic stainless’ ability to resist corrosion. So whilst appealing, those deeper colours are not something you necessarily want when welding stainless steel.
Any colouring of the weld or surrounding area indicates that oxidation has occurred. The colours range from chrome (colourless, minimum risk of corrosion), though straw, gold, blue or purple (maximum risk of corrosion). The darker the colour, the thicker the level of oxidation. In the pharmaceutical industry, straw (light yellow), is the highest acceptable level. In the dairy industry, however, oxidisation up to light blue is often accepted. In most instances, this surface oxidisation can be removed by a number of means, ie. Chemical pickling, mechanical cleanings or grinding, or the workpiece re-dipped to restore its corrosion resistance. Deeper oxidation can be more problematic.
Reducing Oxidation
When welding with MIG or TIG, one of the biggest contributors to oxidation of the surface is the atmosphere coming into contact with the heated weld area. To prevent this, an inert shielding gas is used to protect the heated area until it drops below the critical temperature. The correct flow rate of the shielding gas is required for reducing the possibility of oxidation and gives cleaner welds.
Too high a flow rate can cause vortexing and introduce the atmosphere into the shielding gas. Using a gas lens rather than a diffuser provides better lamellar flow and better protection.
While rainbow weld colours can look great on your latest steel sculpture, hot-rod, or your motorbike’s titanium exhaust pipes, they do not necessarily perform that well in service.
More Information
Our team of experts at Technoweld can help you determine the best parameters for a given welding process. We can offer a variety of services from training in a specific area to recommending a WPS for compliance to standards. Get in contact with us to discuss how we can help.