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This book deals mainly with welding mild steels. But as not all bits found under the bench or rescued from the scrapheap will be made of it, we’ll start with different materials and their weldability. 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. Section 1 expands on what follows, dealing specifically with steel grades and their application (see here).

Wrought iron isn’t very common now, but has been used extensively in farming for chains and hooks. It’s very low in carbon and malleable.

Mild steel is the common user-friendly stuff. It doesn’t harden (much) when heated and cooled, and is easy to bend and weld. Black mild steel is what you’d normally buy: as flat strip and other sections it comes with radiused edges and retains its coating of mill scale from hot-rolling.

Bright mild steel in 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. It’s 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 the carbon steels (Table 1). As carbon level climbs, so does the end product’s hardness, brittleness, and difficulty of welding.

After forming 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 case.

The more carbon in a steel, the harder it is to file — and files themselves have 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. 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 item with those from a chunk of mild steel.

Although heat treatment will improve a carbon steel’s resilience, the 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 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 shafts. Stainless steel is a variant using chromium to beat corrosion, which for the metalworker is both good and bad news. Although stainless steel is slow to tarnish, that reluctance to oxidize means it can’t be gas-cut — but it can’t resist a plasma cutter. And while many stainless steels are non-magnetic and weldable, don’t weld if a magnet sticks to the bit you want to use; cracking is very likely.

Two jobs using a dissimilar steels electrode: a sash cramp’s cast iron endplate welded to the central mild steel beam for more rigidity

A slurry pump’s cast steel shear plate resurfaced back to near-original dimensions.

Table 1: Materials and their weldability

Material	Percentage carbon	Weldability	Typical use
Wrought iron	0.01–0.03	Good	Hooks, chains
Dead mild steel	0.1–0.125	Good	Wire, pressings
Mild steel	0.15–0.3	Good	General engineering
Medium carbon steel	0.3–0.5	May be made brittle	Structural steels, forgings, high tensile tube, some tools
High carbon steel	0.5–0.7	Will be made hard and brittle	Chisels, springs, hammers, railway lines
Very high carbon steel	0.9–2.0	Not weldable on farm	Files, razor blades, axes
Gray cast iron	2.0–4.0	Needs right method	Casting not subject to shock. Housings, pulleys, manifolds, etc.
White cast iron	2.0–4.0	Not weldable	Hard layer in wearing parts, camshafts
Malleable cast iron	2.0–4.0	Good with right method	Cutter bar fingers, coulter brackets, vice parts, clamps, etc.

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 gives the answer.

Gray cast iron breaks very easily if bent or shocked to leave a grainy surface. But 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 gray 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 vice bodies, clamps, and pto shaft yokes. White cast iron is very hard and brittle, properties that are used when a cast part must resist wear. So for some soil-engaging parts, the molten iron is chilled in specific areas while in the mold, forming an outer layer of hard while 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 red sparks that don’t sparkle and fade very close to the wheel, while cast steel sparkles clear yellow like mild — though the sparks are closer to the wheel and more bushy.

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, gray 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 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 gray cast underneath files, drills, and machines very easily. Cast steels don’t have this hard shell.

Iron and Steels: Carbon Contents and Uses

Blast furnaces make pig iron, which is high in carbon and impurities. Refining produces the following series of materials, with hardness and brittleness increasing as carbon content goes up.

Steels in the lower reaches of the carbon league are weldable on the farm. Ditto for those in the middle, though they need greater care in 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 — see Section 2.

What Should You Weld?

Everything depends on the material involved and its application. Making 100% 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% 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 that matches the most awkward of the metals to be joined.

• Preheat. A gas flame heats moderatesized 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 drafts. Don’t put just-welded work on cold surfaces and never, ever, quenchcool. 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 stressrelived after welding by heating the joint area to very dull red and then cooling slowly (see distortion control).

Welding cast iron is covered in Section 4. Preheating gray castings helps a great deal, and low welding heat input followed by slow post-weld cooling are 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 lowertemperature bronze welding is better. Gray cast will turn to the brittle white form if cooled too quickly.