Thursday, November 30, 2017

Sheet Metal Overview


This post is a simple introduction to metals, with a particular focus on carbon steel sheet metal. It covers metal's place in the periodic table, and the common and mechanical properties of all metals. It also covers how iron is mined and processed. It covers how steel is produced from iron, and some common types of sheet metal used in fabrication.

Overview

About three-quarters of the period table is composed of elements which are metal. All those shown below in gray are metal.

Common Properties

Metal elements share several other common properties:
  • They are usually solid at room temperature (mercury is an exception)
  • They are usually shiny.
  • They have a high melting point.
  • They are a good conductor of heat.
  • The are a good conductor of electricity.
  • They are malleable (able to be hammered or pressed permanently out of shape without breaking or cracking). 
  • They are ductile (able to be deformed without losing toughness; pliable, not brittle).
  • They have a high density (exceptions are lithium, potassium, and sodium).
  • They corrode in air or salt-water.
  • They lose electrons in reactions.

Categories: Ferrous and Nonferrous 

There are two main categories of metals used in fabrication. 

Ferrous materials contain iron. Ferrous materials are those primarily found in iron, cast iron, steel and wrought-iron. Other examples are mild steel, medium carbon steel, high carbon steel, stainless steel and high speed steel.

Nonferrous materials do no contain iron, or only extremely small amounts of it. Some examples of nonferrous materials are aluminum, titanium, copper, zinc, and lead.

Ore and Iron 

Metal is mined from the surface of the earth! Iron is the 4th most abundant element on the earth's crust and aluminum is the most abundant metal on the earth's crust. Aluminum, or the ore named Bauxite, comprises 8.1% of the earth's crust. Iron is 5%.

The eight most common elements in earth’s crust, listed by mass, are:
  • 46.6% Oxygen (O)
  • 27.7% Silicon (Si)
  • 8.1% Aluminum (Al)
  • 5.0% Iron (Fe)
  • 3.6% Calcium (Ca)
  • 2.8% Sodium (Na)
  • 2.6% Potassium (K)
  • 2.1% Magnesium (Mg)

Ore is the rock which is mined from the ground from which the metal is extracted. Iron ores are rocks containing iron and other minerals. In certain areas the rocks will contain higher percentages of Iron Oxide - the more iron oxide in the rock the better. The name of the ore depends on the percent content of iron present.
  • Hematite - Fe2O3 - 65% Iron
  • Limonite - Fe2O3+H2O - 50%-65% Iron
  • Taconite - Fe3O3 - 25%-35% Iron
  • Magnetite - Fe3O4 - 22% Iron

Hematite - Image Source


Magnetite - Image Source

Making Iron from Ore

Sometimes the ore is refined by crushing it and concentrating the iron oxide by removing waste rock. This refined ore is made into marble size pellets which are then put into the blast furnace with limestone. The limestone is used as a flux, it melts and removes unwanted impurities in the iron.

The following video is a brief look at the process of extracting iron from ore: 



This next video is a longer (45 minute) and offers a more complete look at the production of iron from ore. It also looks at other uses for iron:



Carbon Steel

Pure, cast iron has a major problem. It's brittle - it breaks easily under impact. In 1856 British engineer Henry Bessemer found a way to make a stronger form of iron. He combined iron with carbon in a blast furnace to make steel. Steel is a a hard, strong, gray or bluish-gray alloy of iron with carbon and usually other elements, used extensively as a structural and fabricating material.

Carbon steel is a steel with carbon content up to 2.1% by weight. As the carbon percentage content rises, steel has the ability to become harder and stronger through heat treating; however, it becomes less ductile. Regardless of the heat treatment, a higher carbon content reduces weld-ability.

Mild or Low-Carbon Steel
Mild steel (steel containing a small percentage of carbon, strong and tough but not readily tempered), also known as plain-carbon steel and low-carbon steel, is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Mild steel contains approximately 0.05–0.25% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form.

Higher-Carbon Steel
Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30–1.70% by weight. These are known as high-carbon steel.

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.

Sheet Metal

Sheet metal is metal formed by an industrial process into thin, flat pieces. Sheet metal is one of the fundamental forms used in metalworking. It can be cut and bent into a variety of shapes. A great many everyday objects are fabricated from sheet metal. Its thicknesses can vary significantly; extremely thin sheets are referred to as foil or leaf, and pieces thicker than 1/4" (6 mm) are considered plate.

There are many different metals that can be made into sheet metal, such as aluminium, brass, copper, steel, tin, nickel and titanium. For decorative uses, some important sheet metals include silver, gold, and platinum.

Gauge

In most of the world, sheet metal thickness is consistently specified in millimeters. In the US, the thickness of sheet metal is commonly specified by a traditional, non-linear measure known as its gauge. The larger the gauge number, the thinner the metal. Commonly used steel sheet metal ranges from 30 gauge to about 7 gauge.

Gauge differs between ferrous metals and nonferrous metals such as aluminum or copper; copper thickness, for example is measured in ounces, which represents the weight of copper contained in an area of one square foot.

Cold Rolled Steel

Cold rolling happens at temperatures that are close to normal room temperature. This increases the strength of the finished product through the use of strain hardening by as much as 20 percent. This steel often has a gray finish that feels smooth to the touch.

The cold rolled process creates a finished product that is more precise dimensionally than a hot rolled steel. This is because it is already closer to the finished dimension since it has already gone through the cooling process.

Hot Rolled Sheet Metal

Hot rolled steel comes from a rolling process which happens at temperatures above 1000 degrees Fahrenheit. The steel actually re-configures itself during the cooling process, giving the finished product looser tolerances than the original material and when compared to cold rolled steel metal. Hot rolled steel is more malleable than cold rolled.

Pickling and Oiling

The surface of hot steel reacts with the oxygen and water vapor in the air, forming something similar to a very heavy rust. This is called scale. The scale needs to be removed, and pickling is the name given to the chemical removal of scale (using acid). Once the steel is cleaned it can be oiled to prevent future contamination.

Common Sheet Metals from Alro

Below is a link to the online Alro Metals site. You can use the link to price sheet metal. For water jet cutting in the FabLab we often use Carbon Steel > Sheet > A1008 CR. You can specify the dimensions and add to the shopping cart to get pricing information. 

Alro Online Store - Steel

Reference

This section provides a few links to general information about sheet metal and a list of terminology of its mechanical properties: 


Mechanical Properties of Metal

The mechanical properties of a material determine its usefulness for a particular application or product. The following terms help to classify material properties into categories which can be tested. 

Strength: The strength of a material is its ability to resist changing its size or shape when an external force is applied to it.
Malleability: Malleability is the property by which a metal can be rolled into thin sheets.
Ductility: Ductility is the property by which a metal can be drawn into thin wires.
Hardness: Hardness is the ability of material to resist permanent change of shape caused by an external force.
Brittleness: The tendency of material to fracture or fail upon the application of a relatively small amount of force or impact.
Elasticity: Elasticity is the tendency of a solid material to return to its original shape after being deformed.
Creep: When a metal is subjected to a constant force at a high temperature below its yield point, for a prolonged period of time, it undergoes a permanent deformation called creep.
Fatigue: Fatigue is the of material weakening or breakdown of equipment subjected to stress, especially a repeated series of stresses.
Plasticity: Plasticity is the property by which a metal retains its deformation permanently, when the external force applied is released.
Resilience: Resilience is the ability of metal to absorb energy and resist soft and impact load.
Stiffness: When an external force is applied on metal, it develops an internal resistance. The internal resistance developed per unit area is called stress. Stiffness is the ability of metal to resist deformation under stress.
Toughness: When a huge external force is applied on metal, the metal will experience a fracture. Toughness is the ability of metal to resist fracture.
Yield Strength: The ability of metal to bear gradual progressive force without permanent deformation.
Stress: The load per unit area. Types of stress include shear, tensile and compressive stress.
Strain: The unit deformation of a metal when stress is applied.

Welding

The following video discusses many aspects of welding: