Alloying Elements

Sign Effect on Steel Mechanical and

Corrosion Properties

Carbon C

Necessary to convert iron in to Steel and allow hardening and increase in strength increase tensile strength and hardness but reduces toughness. Reduce weldability. Increase corrosion. Restricted to 0.030% in stainless steel ( L-grades ) to reduce sensitization during welding

Manganese Mn.

Improve hot working properties and increases tensile strength , hardness and abrasion resistance. Austenite Forming element. Decrease porosity and cracking forms sulphides that may cause hydrogen induced cracking

Phosphorous P

Increase brittleness and cracking during welding. Increase being restricted to < 0.025 % for sweet and < 0.015 % for sour service pipe. Added with sulphur to steel to improve machinability but such materials should not be used for submarine service.

Alloying Elements

Sign Effect on Steel Mechanical and

Corrosion Properties

Sulphur S

Increase Porosity, brittleness and cracking by hot Shortness during forming forms manganese sulphide thattrap hydrogen leading to internal cracking. Surface emergent sulphides initiate pitting. Increasingly restricted to 0.01 % Sweet and < 0.005 % for sour Pipe line.

Selenium Se.Added to improve machinability. Not use for pipe line Steels.

Silicone SiIncrease tensile strength but markedly reduce toughness. Added as a deoxidizer to kill the steel ( removal gases ). Restricted to 0.35 – 0.4 %

Aluminium Al

Used to refine grain size. Increase harness. Added as a deoxidizer to kill steel. Add weld toughness when added to 0.02 – 0.05 %

Alloying Elements

Sign Effect on Steel Mechanical and

Corrosion Properties

Copper Cu

Improves sour cracking resistance for improvements at pH>4.5. Affects corrosivity of weld HAZ. In conjunction with Ni claimed to stabilized corrosion films and reduce corrosion. Often used with NI for pipe line for bends and in thick section pipe. Tramp elements in stainless steels though used to increase hardness in special alloys.

Calcium Ca.Deoxidiser and desulphuriser. Secondary addition used for inclusion shape control for sour service pipe steel.

Chromium Cr.

Used to improve resistance to oxidation. Increase tensile strength and hardness. Decreases weldability. Has major effect on corrosion resistance. Material become a stainless steel if Cr ≥ 12 %

Cobalt CoTramp element. Become radioactive when irradiated so generally minimized.

Alloying Elements

Sign Effect on Steel Mechanical and

Corrosion Properties

Molybdenum Mo

Increase tensile strength and corrosion resistance . Reduce Pitting attack when added to chromium – Nickel stainless steel. Used in High grade bends as it improves hot hardness.

Titanium Ti.Micro alloying element. Increase tensile strength, hardenability and wear resistance. Combines with carbon to form carbides that may reduce toughness. Used in stainless steel to prevent sensitization.

Nitrogen NAustenite stabiliser. Increase strength but reduce low temperature toughness. Used to increase strength of austenitic ( N grade ) and Duplex stainless steel. Restricted to 0.09 to 0.15 %

Niobium Nb.Micro-alloying elements in C-Steel and always added to steel above X42. used to stabilised carbon in stainless steel to prevent sensitision.

Vanadium Vn.Increase tensile strength , hardenability and wear resistance . Used micro alloying element for thick pipeline material.