How to Weld: Metal Identification Basics

Before you learn welding techniques, you need a basic understanding of the materials you’ll be working with.
By Andrew Pearce
November 16, 2012
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Blast furnaces make pig iron, which is high in carbon and impurities. Hardness and brittleness increases as carbon content goes up. Steels in the lower reaches of the carbon league are weldable on the farm. So are those in the middle, though they need greater care over rod selection, joint preparation and subsequent cooling. High carbon steels are unweldable by normal methods. Adding dashes of other elements to carbon steels gives a wide range of tougher alloy steels.  
Chart Courtesy Fox Chapel Publishing
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Almost anyone can learn the craft of welding, Andrew Pearce argues in his straightforward and handy guide to do-it-yourself metal work, Farm and Workshop Welding (Fox Chapel Publishing, 2012). In this excerpt from the book’s introduction, Pearce gets things started by explaining the compositions of different alloys and the properties of different metals. 

Buy this book from the MOTHER EARTH NEWS store: Farm and Workshop Welding.

What Is Welding?

It’s the process of joining materials using heat. In fusion welding, joint components are heated until they melt together or are positively fused by pressure. Blacksmiths use heat and hammer blows, but here we’re more concerned with getting heat alone to do the work.

This heat will come from either an electric arc, a gas flame, or in the case of plastics, from a hot air gun. Filler is usually added to the joint from an electrode or separate rod. Non-fusion welding techniques like braze (or bronze) welding and soldering use heat too, but not enough to melt the metals that form the joint.

Metal Identification

Although accurate identification of steel is a complex business, the main classes can be sorted out with a file, a grinder and some basic ground rules. Wrought iron is no longer very common, but in the past has been used extensively for chains and hooks. It’s very low in carbon, and malleable. Mild steel is the common user-friendly stuff. It doesn’t usually harden when heated and cooled, and is easy to bend and weld. Black mild steel is what you’d normally buy: as strip it comes with rounded edges and retains its coating of mill scale from hot-rolling. Bright mild steel in its flat form has square edges, is shiny and is more accurately sized than mild steel. It’s made by cleaning and cold-rolling black mild steel, leaving the metal stronger but less ductile.

Silver steel looks like bright steel but is much harder. It contains chromium but, oddly, no silver and is usually sold in short lengths. Black and bright mild steels are easily filed and give off long, light yellow sparks under an angle grinder. Both are readily weldable. Silver steel is not.

Adding more carbon to steel makes it harder, and logically enough, produces carbon steels. As the carbon level climbs, so does the end product’s hardness, brittleness and difficulty of welding.

After being formed to shape, carbon steels are often heat-treated (tempered) to boost their resilience. Welding heat can destroy the tempering effect, leaving the joint zone hard and brittle until it’s re-treated. Springs are a classic example.

The more carbon in steel, the harder it is to file — and files themselves have a very high carbon content. So here’s a quick test. If an unknown material can’t be filed, it’s probably not weldable.

The exception can be cast iron; see below. The grinding spark pattern also changes with carbon level. As it rises, the sparks get shorter, bush out closer to the grinding wheel and may be darker yellow in color. If in doubt, compare sparks from the unknown metal with those from a chunk of mild steel.

Although heat treatment will improve the resilience of carbon steel, really spectacular gains come from adding small quantities of exotic elements to produce alloy steels. All sorts of metals — nickel, tungsten, manganese, molybdenum, cobalt, vanadium — can spice up the mix, and the end result is usually heat-treated to maximize its properties. Alloy steels turn up wherever toughness, resilience and corrosion resistance is needed. Typical applications are springs, gears and transmission half-shafts. Stainless steel is a variant using chromium to beat corrosion, which for the metalworker is both good and bad news. Although stainless is slow to tarnish, that reluctance to oxidize means it can’t be gas-cut. While many stainless steels are non-magnetic and weldable, if a magnet sticks to the bit you want to use don’t try welding it – cracking is very likely.

Sorting an alloy from a carbon steel is largely a matter of application, though stainless stands out readily enough thanks to its satiny bright finish. Think about cost too: a cheap hand tool is more likely to get its hardness from a tempered carbon steel than an expensive alloy one. Castings can be recognized by their complex shapes, generally rough surface finish and any raised surface lettering. But is the bit in your hand cast iron or cast steel? Application and a grinding test usually supply the answer.

Grey cast iron breaks very easily if bent or shocked to leave a grainy surface. Yet it stands compression loads very well, so turns up in machine beds, bearing housings, electric motor bodies, belt pulleys, engine blocks, manifolds and such. Heat treating grey cast iron produces the much tougher malleable cast iron, which is close to mild steel in strength and ductility. Malleable cast is used where shock loads are high; in vise bodies, clamps and PTO shaft yokes. White cast iron is very hard and brittle, properties which are used when a cast part must resist wear. So, for some soil-engaging parts, the molten iron may still be chilled in specific areas while in the mold, forming an outer layer of hard white cast.

Cast steels stand much harder service, being tougher than cast irons and capable of being heat treated to boost their resilience. Cast steels turn up where a durable complex shape is called for.

Telling the two apart is pretty simple. The quickest way is to grind them: cast irons give off unmistakable dull red/orange sparks that don’t sparkle, and fade very close to the wheel, while cast steel sparkles clear yellow like mild steel — though the sparks are closer to the wheel and bushier.

The hammer test is another decider. Tap cast steel and it rings, while cast iron just makes a dull clunk. Other differences? Cast iron fractures to leave a very characteristic coarse grainy grey surface — break a bit to see — and if you drill or file it, the swarf is powdery. Cast steel produces silvery filings.

When you start to file or machine some cast iron it may seem very tough. This is down to a hard skin of white cast iron, formed on the surface where molten iron contacted cold sand in the mold. Break through this skin and the grey cast underneath files, drills and machines very easily. Cast steels don’t have this hard shell.

What Should You Weld?

Everything depends on the material and its application. Making 100-percent reliable joints in anything other than mild steel needs the right electrode and technique, and may call for specific procedures before and after welding to retain the metal’s properties.

There is only one rule. Don’t weld any safety-related component unless you’re completely sure about its makeup and any heat treatment it may have had. If the part must be repaired rather than replaced, take it to a specialist.

What are the options when safety is not at stake, or 100-percent reliability is not essential? Here a “dissimilar steels” electrode may be the answer. Although metallurgists rightly stress the importance of matching rod and material, these jack-of-all-trades rods often get round material mismatches. If you’re faced with joining carbon or alloy steel:

• Choose a rod which matches the most awkward of the metals to be joined.
• Preheat. A gas flame heats moderate-sized parts. Move it around to keep heat input even.
• Use the minimum current needed for fusion, and keep run number low.

After welding, let the work cool very slowly. Lay it on warmed firebricks or on dry sand and cover it to keep off draughts. Don’t put just-welded work on cold surfaces and never, ever, quench-cool. Even mild steel can harden a little if its carbon content is toward the upper limit, so where strength really matters, don’t quench a mild steel repair in water.

Medium carbon steels can be stress-relieved after welding by heating the joint area to very dull red and then cooling slowly.

Preheating grey castings helps a great deal, and low welding heat input followed by slow post-weld cooling is always necessary. Even then, success with cast iron is never completely certain thanks to the material’s tendency to crack as it cools. It’s important to know which cast iron you’re dealing with: malleable cast will cool to brittleness if arc welded, so lower-temperature bronze welding is better. Grey cast will turn to the brittle white form if cooled too quickly.

Ready to get started? Learn more about how to weld with How to Solder: Great Tips on Soldering for Beginners, another excerpt from Farm and Workshop Welding.


This excerpt has been reprinted with permission from Farm and Workshop Welding by Andrew Pearce, published by Fox Chapel Publishing, 2012. Buy this book from our store: Farm and Workshop Welding. 


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