Have you ever wondered what all those elements listed on a materials test report really mean? Read on!
Carbon is generally considered to be the most important alloying element in steel and can be present up to 2% (although most welded steels have less than 0.5%). Increased amounts of carbon increase hardness and tensile strength, as well as response to heat treatment (hardenability). Increased amounts of carbon will reduce weldability.
Sulfur is usually an undesirable impurity in steel rather than an alloying element. In amounts exceeding 0.05% it tends to cause brittleness and reduce weldability. Alloying additions of sulfur in amounts from 0.10% to 0.30% will tend to improve the machinability of a steel. Such types may be referred to as “resulfurized” or “freemachining”. Free-machining alloys are not intended for use where welding is required.
Phosphorus is generally considered to be an undesirable impurity in steels. It is normally found in amounts up to 0.04% in most carbon steels. In hardened steels, it may tend to cause embrittlement. In low-alloy high-strength steels, phosphorus may be added in amounts up to 0.10% to improve strength and corrosion resistance.
Usually only small amounts (0.20%) of silicon are present in rolled steel when it is used as a deoxidizer. However, in steel castings, 0.35 to 1.00% is commonly present. Silicon dissolves in iron and tends to strengthen it. Weld metal usually contains approximately 0.50% silicon as a deoxidizer. Some filler metals may contain up to 1% to provide enhanced cleaning and deoxidation for welding on contaminated surfaces. When these filler metals are used for welding on clean surfaces, the resulting weld metal strength will be markedly increased. The resulting decrease in ductility could resent cracking problems.
Steels usually contain at least 0.30% manganese because it assists in the deoxidation of the steel, prevents the formation of iron sulfide and inclusions, and promotes greater strength by increasing the hardenability of the steel. Amounts of up to 1.5% can be found in some carbon steels.
Chromium is a powerful alloying element in steel. It strongly increases the hardenability of steel, and markedly improves the corrosion resistance of alloys in oxidizing media. Its presence in some steels could cause excessive hardness and cracking in and adjacent to welds. Stainless steels may contain in excess of 12% chromium.
Molybdenum is a strong carbide former and is usually present in alloy steels in amounts less than 1%. It increases hardenability and elevated temperature strength. In austenitic stainless steels it improves pitting corrosion resistance.
Nickel is added to steels to increase hardenability. It often improves the toughness and ductility of the steel, even with the increased strength and hardness it brings. It is frequently used to improve toughness at low temperature.
Aluminum is added to steel in very small amounts as a deoxidizer. It also is a grain refiner for improved toughness; steels with moderate aluminum additions have been made to a “fine grain practice”.
The addition of vanadium will result in an increase in the hardenability of a steel. It is very effective, so it is added in minute amounts. At greater than 0.05%, there may be a tendency for the steel to become embrittled during thermal stress relief treatments.
General Steel Definitions
The terms and conditions of each manufacturer’s guarantees will vary. Most manufacturers repair, replace or correct any defective material or workmanship for varying periods of time. Many steel tanks built over 60 years ago are still in use today. TI1is testifies to the effectiveness, reliability and economy of steel structures. Proper maintenance, improved paints, and cathodic protection systems have greatly increased the life expectancy of steel tanks.
0.15% maximum carbon, used for electrodes, plate and shapes, sheet, strip, excellent weldability.
0.15-0.30% maximum carbon, used for structural shapes, plates and bars, good weldability.
0.30-0.50% maximum carbon, used for machinery parts, fair weldability – preheat and postheat may be needed.
0.50-1.00% maximum carbon, used for springs, dies, rails, poor weldability – difficult to weld without preheat and postheat.
Thank you to the American Welding Society for providing some information used in this resource.
April 1, 2016
- Pressure Vessels