Welding Reference Library · Measure & Identify

Identifying Base Metals in the Field

Quick shop tests (magnet, color and weight, file, spark and chip) for sorting an unknown base metal into a family before choosing a filler, and their limits.

Ref WG-0311

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Customers often bring in a part and ask what rod or wire to use. The first step is to sort the metal into a family: carbon steel, cast iron, stainless, aluminum, copper alloy. Simple shop tests do that well enough to start a conversation. They cannot give an alloy grade. When the grade matters, use mill certs, drawings, or positive material identification (PMI) with an XRF or OES analyzer.

How to identify

Look and weigh. Color, surface finish and how heavy the part feels for its size.
Magnet. Strongly attracted, weakly attracted or not at all.
File. Does a file bite easily, skate, or chip off?
Spark test. Touch to a grinding wheel and watch the spark stream against a dark background. Wear eye and face protection.
Chip or fracture. Look at a chip from a chisel or a broken edge.

Field test chart

Metal family	Magnet	Look and feel	Spark test	Other tells
Low carbon steel	Strong	Gray, rusts red-brown	Long straw-colored streaks, few bursts	File bites easily
High carbon or tool steel	Strong	Gray	Brighter stream with many bursts	File may skate on hardened parts
Gray cast iron	Strong	Dull gray, grainy fracture	Short dull red streaks with small bursts	Chips break off as small pieces
Austenitic stainless (304, 316)	None or slight (more where cold worked)	Bright, does not rust	Shorter streaks, few bursts	Heavy like steel
Ferritic or martensitic stainless (400 series)	Strong	Bright, may show light rust	Similar to stainless above	Magnet alone can mislead
Aluminum	None	Light weight, white-gray, white oxide	No sparks	Very soft to a file; leaves a smear
Copper	None	Red-brown, green patina	No sparks	Heavy, soft
Brass and bronze	None	Yellow to reddish gold	No sparks	Heavy
Manganese steel	None or slight	Gray, often wear parts	Bright with bursts	Work hardens; file skates

Key points

Spark testing takes practice. Compare to known samples on the same wheel.
A magnet alone cannot separate 400-series stainless from carbon steel.
Cast iron, high carbon steel and hardened parts usually need preheat and special fillers. Confirm before welding.
Plated, painted and galvanized parts hide the base metal and produce hazardous fumes. Clean a test area first.
Grind aluminum and magnesium on a separate wheel, not a steel wheel.

Common mistakes

Calling any nonmagnetic bright metal 304.
Spark testing aluminum on a steel wheel.
Assuming a casting is cast iron when it may be cast steel.
Choosing a filler for code work without a verified grade.

Governing standards: No single governing standard. Use mill certificates and PMI where the grade matters.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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