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Would You Please Pass The Cobalt…an idiot’s guide to the chemistry of steel

May 23rd, 2016 by Mike Meyerhoff

I sat next to the bad kids’ table at lunch in middle school.  One day I overheard one of them describe how their big brother blew up a mail box with some dry-ice and a soda bottle…thus began my fascination with chemistry.  Since then I have spent countless hours toiling over stoichiometric equations, dyed my hands brown by accident for ten days, learned about exothermic reactions the hard way, and twice used the safety shower at the university chem lab.  So as to save you the tortures of chem class, in an effort to share my love of the things that make up other things, here’s a break down of the popular alloying elements found in steel.

 

The Basics 

Steel is iron metal alloyed with carbon.  Four-thousand years ago people realized that if they added certain ingredients to their molten iron in certain proportions, their knives, spears, and swords would end up being stronger, lighter, sharper, so on and so forth.  This iron mixed (alloyed) with carbon based ingredients (like coal) was quite popular and thus STEEL was born.

Over the years as our technology has advanced we have learned (and continue to learn) how to develop new metal alloys which in turn give us the ability to manufacture the new products which till our fields, hold up our buildings, send us to space…via Russia, and absurdly rocket baseballs of collegiate bats.  Through very specific processes based on the desired properties of the finished product, certain elements are added to the steel like ingredients in a cake.

Below is a list of the more common alloy elements found in modern steel products, as well as a general description of their effect on the finished product.  You’ll see that many of them seem to do the same thing; long-story-short, its’ all about how each individual element specifically interacts with any other element.

 

Low-carbon – 0.15% maximum carbon, used for electrodes, plate and shapes, sheet, strip, excellent weldability.

Mild steel – 0.15-0.30% maximum carbon, used for structural shapes, plates and bars, good weldability.

Medium carbon – 0.30-0.50% maximum carbon, used for machinery parts, fair weldability – preheat and post-heat may be needed.

High carbon – 0.50-1.00% maximum carbon, used for springs, dies, rails, poor weldability – difficult to weld without preheat and post-heat.

 

Iron (Fe)        Iron is not an alloy element.  It’s the base element for steel however so I thought it at the very least deserved a shout-out.  None of the other discussion boards or scholarly articles on Google even mention iron so I though we could be the first…

 

Carbon (C)       Carbon is the primary alloy element in steel, and responsible for most of the hardening.  It can account for as much as 2% of the steel’s content.  Hardness and tensile strength of steel increases with an increase of carbon content, while ductility and weldability decrease.

 

Phosphorus (P)        Phosphorus is generally considered to be an undesirable impurity in steels. It is normally found in amounts limited 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.

 

Manganese (Mn)        Steels usually contain at least 0.30% manganese because it assists in surface quality of the steel, counteracts the brittleness from sulfur (prevention Iron Sulfide build-ups), and promotes greater strength by increasing the hardenability of the steel. Amounts of up to 1.5% can be found in some carbon steels.  Manganese’s effect on steel’s strength is dependent on the relative carbon content and is commonly added to high carbon steels to promote abrasion resistance.  Like carbon, an increase in Mn leads to a decrease in weldability and ductility.

 

Aluminum (Al)        Aluminum helps as a deoxidizer…that means it helps prevent rust.  It also has a major effect on controlling grain growth.  Grain…in steel…wha???  Yeah…it’s a whole other deal which I’ll write about next time business is slow.

 

Sulfur (S)        Sulfur sucks in steel… you don’t want it!  Too much Sulphur in the steel is want sank the Titanic!  Sulphur mixed with steel makes iron sulfide, which cools at a different temperature creating weak and brittle pockets in the steel…the kind which snap easily in cold north Atlantic water.  Many steels limit its content to less than 0.05%.  It can help with machinability though so some proprietary steels allow for Sulphur contents up to 0.35%, but in those cases increased manganese is required to counter the negative effects of the Sulphur.

 

Silicon (Si)        Generally speaking, Silicon is what makes up rocks…earth is a rocky planet…is considered a silicate planet.  Si can act much like Mn, but its less effective and can be detrimental to surface quality in low carbon steels especially.  Typicaly Si is used in small amounts (0.20%) in rolled steel as a deoxidizer (rust preventer), but is used in higher concentrations (0.35 – 1.00%) in castings to strengthen the steel.  Si is used as an alloy element in electrical and magnetic steels.

 

Chromium (Cr)        Chromium is the most important element in stainless steels due to its corrosion resistant properties, and can account for more than 12.00% of the steel’s content.  In carbon steels it also increases hardenability, improves high temperature strength, and is thus common in abrasion resistant steels.

 

Cobalt (Co)        Cobalt basically gets iron to harden as it solidifies.  It also adds tempering resistance so its useful in high temperature applications.  Co is more common in higher grade steels.

 

Molybdenum (Mo)        This stuff significantly increases the resistance to both uniform and localized corrosion, as well as general hardness and creep strength at elevated temperatures.  By the way, “creep” is a term to describe how a solid can move or deform over time due to stress…learn something new every day.

 

Nickel (Ni)        Nickel generally increases ductility and toughness and is one of the main alloy elements in stainless steels. It also reduces the corrosion rate and is therefore advantageous in acidic environments. In martensitic grades adding nickel, combined with reducing carbon content, improves weldability.  Nickel is frequently used to improve toughness at low temperatures.

 

Vanadium (V)        Vanadium is very effective in increasing the tensile and yield strength of carbon steels.  Used in concentrations greater than 0.05%, V may cause embrittlement.   This stuff works by promoting finer grains (increasing toughness) and  it resists tempering and causes secondary hardening.

 

Copper (Cu)        Copper is the main alloying element in corten steel (in concentrations greater than 0.20%). Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%.

 

Lead (Pb)        It is almost insoluble in liquid or solid steel.  However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.

 

http://www.outokumpu.com/en/products-properties/more-stainless/the-effects-of-alloying-elements%E2%80%8B/Pages/default.aspx#active

http://www.aws.org/

http://ispatguru.com/alloying-elements-and-their-influence-on-properties-of-steel/

http://metallics.org.uk/the-effects-of-alloying-elements-on-steel/

http://elmtreeforge.blogspot.com/2005/04/problem-of-sulphur-in-steel.html

http://steel.keytometals.com/articles/art50.htm