Base Metal Identification
Metal families and weldability
Carbon steels (A36, 1018) weld readily. Low-alloy and high-strength steels (4140, AR plate, HY-series) demand preheat and low-hydrogen practice. Austenitic stainless (304, 316) needs matched stainless filler and dedicated tooling. Cast iron is welded only with specific procedures - preheat, short stringer beads, nickel or nickel-iron electrode. Coated steels (galvanised) require coating removal and fume control.
Weldability follows carbon equivalent: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Below about 0.4, normal practice is usually adequate; above 0.4, preheat, low-hydrogen electrodes and controlled heat input become mandatory.
Field tests, in order of reliability
- Mill test certificates or a material test report - always the first choice.
- Spark test: low carbon gives long, straw-yellow carrier lines with few bursts; higher carbon gives short, bright, bursting sparks; cast iron gives short, dull red sparks.
- Magnet: ferritic and martensitic steels are strongly magnetic; austenitic stainless is weakly or non-magnetic - but cold-worked 304 becomes magnetic, so this test proves little on its own.
- Fracture or chip test on a scrap corner; file and hardness checks for hardness level.
Worked example
An unmarked shaft sparks short and bright, skates a file and is strongly magnetic. Treat it as a medium-carbon or low-alloy steel. Estimating 4140 chemistry gives CE = 0.40 + 0.85/6 + 1.0/5 + 0.2/5, about 0.78. Action: obtain certificates, preheat 200-300 degrees C, use E7018 low hydrogen, stringer beads, controlled heat input, slow cooling.
Traps
- Treating a magnet test as proof of steel type.
- Welding an unknown alloy with a rutile electrode and no preheat.