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How To Cut With Oxyacetylene, Part 5

Occasionally, farmers get a chance to pick up used metal at places like farm auctions. Often, it sells for much less than it would new from a retailer. And having a good assortment of steel around the farm can mean the difference between having the right piece on hand for a quick repair or a long field delay. But how do you know exactly what you’re getting when you buy metal of unknown origin? There are many different types, and they don’t come with stuck-on labels.

Here are a couple of tips to help you figure out just what you have and how it will react when welded.

In this segment we continue to draw on technical expertise from a variety of sources. They include Cal Shaw, a welding instructor who teaches at SIAST’s Palliser campus, published text books, and technical training material.


Steel is an alloy of iron which contains carbon and other elements. Most machinery and structures use mild carbon steel. Mild — or low carbon — steel has a carbon content of 0.3 per cent or less. But there are other ferrous (iron-based) metals with much higher amounts, and they can look very similar to mild steel. The range of carbon content climbs as steel is manufactured for certain applications that need a much harder structure. For example, cast iron has 1.8 to four per cent carbon.

These different carbon contents require very different welding techniques, so it’s important to make sure you know what you’re working with. Here are two quick tests to help you find out.


The first test involves looking at the spark pattern created by a grinding disc. All it takes is a light touch. The visible difference in the spark pattern created will give you an indication of the amount of carbon the steel contains.

The principle behind the test is that as ferrous metal is heated, different parts of it oxidize at different rates, and the oxidation colours are different. When heated by the friction of the grinding wheel, nearly pure iron, which has a very low carbon content, does not oxidize quickly. So the trail of sparks created by the grinder are long and gradually fade out as they cool (See Graphic 1). As carbon content increases, the characteristics of spark trails change, including their colour, length and behaviour.

Mild steel given a grinder test will show a large volume of long, white spark trails that can jump about 70 inches. Some of the sparks will spurt (explode), shooting off other, smaller sparks at 45 degree angles to the original direction. As the carbon content of the steel increases, the number of spurts, or explosions, increases and the length of trails decrease.

For example, white cast iron will produce a very small volume that extends only about 20 inches. The spark colour close to the grinding wheel will be red and eventually turn to a straw colour near the end of the stream. Grey cast iron will produce a similar, slightly longer stream, but it will have many more repeating spurts.


However, even after passing a spark test indicating they have the right carbon content for common use, some specialized metals may not have good welding properties because of the impurities incorporated into them during the manufacturing process. Welds made on some of these alloys will always crack or fail. A quick oxyacetylene torch test will determine if they will weld properly.

Using an oxyacetylene torch, simply melt a small pool at one place on the metal and watch how it reacts. The molten puddle shouldn’t spark excessively or boil. Then let it cool. If looks shiny after cooling, it is ordinary mild steel. You can be confident it will react well to welding.

When the puddle solidifies, it should have a smooth surface and not be porous or rough. If it looks distorted, it has a higher carbon content, which will make it difficult to weld. If the molten metal has a coloured or dull appearance, that is another indication its unsuitable for welding. If metal doesn’t pass this test, it’s not of much use around the farm.

So before you use some of that bargain metal you picked up at the last auction, take a few minutes to test it and make sure it will do the job, particularly if it will be a structural component.


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Use the images on this chart as a basis to interpret the spark pattern given off by a grinder when trying to

determine the type of steel you have.


Wrought Iron

Mild SteelCarbon Tool Steel Gray Cast IronWhite Cast IronAnnealed Malleable Iron

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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