Metal Identification: How to ID Metals

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Metal Identification: How to ID Metals

Summary:

Welcome to the world of Metal ID or identification. The following pages will walk you through the various tests, physical and mechanical properties that are used to determine the origin of metals.

Most of the metals and alloys described in this section of the site can be
welded by one or more of major welding processes (Stick, TIG, MIG, Oxyfuel). Metals need to be identified before welding in order to pick the correct electrode and method. For example, magnesium and aluminum have a similar appearance, but require different welding processes.

This
section describes the characteristics of metals and their alloys, with
particular reference to their significance in welding operations.

All metals fall within two categories, ferrous or nonferrous.

  1. Ferrous metals are metals that contain iron.
    Ferrous metals appear in the form of cast iron, carbon steel, and tool
    steel. The various alloys of iron, after undergoing certain processes,
    are pig iron, gray cast iron, white iron, white cast iron, malleable
    cast iron, wrought iron, alloy steel, and carbon steel. All these types
    of iron are mixtures of iron and carbon, manganese, sulfur, silicon, and
    phosphorous. Other elements are also present, but in amounts that do
    not appreciably affect the characteristics of the metal.
  2. Nonferrous metals are those which do not contain iron.
    Aluminum, copper, magnesium, and titanium alloys are among those metals
    which belong to this group.

Nonferrous and Ferrous Metals

Copper pipe (nonferrous, left) and Iron Stove (Ferrous metals – right)

Physical Properties of Metals

Many of the physical properties of metals determine if and
how they can be welded and how they will perform in service. Physical
properties which is comprised of several Metal ID methods,  are shown in table 7-1 a&b below.

Physical Properties of Metals - Table 7-1a and 7-1b"

Metal Color

Color relates to the quality of light reflected from the metal.

Mass or Density

Mass or density relates to mass with respect to volume. Commonly known as specific gravity,
this property is the ratio of the mass of a given volume of the metal
to the mass of the same volume of water at a specified temperature,
usually 39°F (4°C). For example, the ratio of weight of one cubic foot
of water to one cubic foot of cast iron is the specific gravity of cast
iron. This property is measured by grams per cubic millimeter or
centimeter in the metric system.

Melting Point

The melting point of a metal is important with regard to welding. A
metal’s fusibility is related to its melting point, the temperature at
which the metal changes from a solid to a molten state. Pure substances
have a sharp melting point and pass from a solid state to a liquid
without a change in temperature. During this process, however, there is
an absorption of heat during melting and a liberation of heat during
freezing. The absorption or release of thermal energy when a substance
changes state is called its latent heat.

Mercury is the only
common metal that is in its molten state at normal room temperature.
Metals having low melting temperatures can be welded with lower
temperature heat sources. The soldering and brazing processes utilize
low-temperature metals to join metals having higher melting
temperatures.

Boiling Point

Boiling point is also an important factor in welding. The boiling point
is the temperature at which the metal changes from the liquid state to
the vapor state. Some metals, when exposed to the heat of an arc, will
vaporize.

Conductivity

Thermal and electrical conductivity relate to the metal’s ability to conduct or transfer heat and electricity.

  • Thermal conductivity,
    the ability of a metal to transmit heat throughout its mass, is of
    vital importance in welding, since one metal may transmit heat from the
    welding area much more quickly than another. The thermal conductivity of
    a metal indicates the need for preheating and the size of heat source
    required. Thermal conductivity is usually related to copper. Copper has
    the highest thermal conductivity of the common metals, exceeded only by
    silver. Aluminum has approximately half the thermal conductivity of
    copper, and steels have abut one-tenth the conductivity of copper.
    Thermal conductivity is measured in calories per square centimeter per
    second per degree Celsius.
  • Electrical conductivity is the
    capacity of metal to conduct an electric current. A measure of
    electrical conductivity is provided by the ability of a metal to conduct
    the passage of electrical current. Its opposite is resistivity, which
    is measured in micro-ohms per cubic centimeter at a standardize
    temperature, usually 20°C. Electrical conductivity is usually considered
    as a percentage and is related to copper or silver. Temperature bears
    an important part in this property. As temperature of a metal increases,
    its conductivity decreases. This property is particularly important to
    resistance welding and to electrical circuits.

Coefficient of Linear Thermal Expansion

With few exceptions, solids expand when they are heated and contract
when they are cooled. The coefficient of linear thermal expansion is a
measure of the linear increase per unit length based on the change in
temperature of the metal. Expansion is the increase in the dimension of a
metal caused by heat. The expansion of a metal in a longitudinal
direction is known as the linear expansion. The coefficient of linear
expansion is expressed as the linear expansion per unit length for one
degree of temperature increase. When metals increase in size, they
increase not only in length but also in breadth and thickness. This is
called volumetric expansion.

The coefficient of linear and
volumetric expansion varies over a wide range for different metals.
Aluminum has the greatest coefficient of expansion, expanding almost
twice as much as steel for the same temperature change. This is
important for welding with respect to warpage, wapage control and
fixturing, and for welding together dissimilar metals.

Corrosion Resistance

Corrosion resistance is the resistance to eating or wearing away by air, moisture, or other agents.

c. Mechanical Properties. The mechanical properties of metals determine the range of usefulness of the metal and establish the serv

For Additional Reading on Metal ID Techniques

Introduction to Metal Identification Author: Jack McHargue, University of Idaho (Free PDF Download)

Metal Identification Basics