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 which 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.
Iron and Steels – Carbon Contents and Uses
Blast furnaces make pig iron, which is high in carbon and impurities. Refining produces the series of materials in the table above, with hardness and brittleness increasing 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.
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% 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 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. (See Distortion Control, page 71.)
Welding cast iron is covered in Section 6. 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.
Section 1
Shielded Metal Arc Welding (SMAW)
Shielded Metal Arc Welding (SMAW, or ‘stick’) is still probably the most common welding technique on the farm. Best suited to materials thicker than 1/8" (3mm), it scores through equipment portability, a relative insensitivity to wind when working outside and a relatively high tolerance of contamination. These relativities will become clear later on. Stick welding is not the fastest technique, but done properly it produces strong joints in a range of thick materials.
What makes a good fusion weld? Pictures 1.1-1.3 show the basics. Achieving fusion and penetration are the key to sound work in any process: fall down on either and joint strength will be lacking. Four runs (or passes) went into picture 1.2. See how weld metal smoothly fuses into the original (or parent) plates, how it penetrates through them and how the individual runs blend together? The result is plenty of strength in the joint. Where the weld metal and parent plate fuse is a heat-affected zone (1.2, D). Here coarse metal crystals form as the area cools, leaving a relatively brittle band where a weld will often fail. Heat treatment and slow cooling after welding can encourage these coarse grains to reform into finer, stronger ones.
1.1 A solid weld – what you’re aiming for. Spot the even U-shaped ripples, full fusion at the edges and the lack of spatter? Good welds come from getting right the four variables (page 20).
This can be seen in 1.2, where the joint is actually made from four weld runs or passes. See how only the top one keeps its coarse grain structure? Grain in the one below has been refined (normalized) by the heat of the following weld pass. That’s why making a joint with several smaller welds often produces a stronger result than just one big run.
Picture 1.3 shows what’s happening during a good SMAW weld. An arc spans the gap between the electrode and the work, generating temperatures of about 9,032°F (5,000°C). Which is one reason why you can’t SMAW-weld metals with a low melting point, like lead or zinc; they can’t stand the heat. During the weld the rod coating burns, producing gas to shield the liquid weld pool from atmospheric contamination and ionized particles to keep the arc running smoothly. At the same time, flux from the coating floats weld impurities to the surface, where it solidifies as a crust of slag. Molten droplets of core wire are pulled across the arc by magnetic force and surface tension, adding extra metal to the joint. Which is just as well or you couldn’t weld uphill, downhill or overhead.
1.2. Sectioning the joint in (1) shows how weld metal (A) has fused with the two parent plates (B) and penetrated through them (C). Good fusion and penetration underpin every strong, durable weld. Notice how metal is built up slightly at the top and bottom of the joint to give a convex profile — a smooth blend at the surface where weld and parent metals join avoids stress concentration, reducing the chance of failure. Where weld and parent plate fuse is a heat-affected zone (D). Here coarser metal crystals grow as the weld cools, leaving a relatively brittle area where a weld will often fail.
1.3. This is what was going on at the rod tip as weld 1.1 was made.