Steel can be classified according to, American’s Standard % age of Carbon content
The Society of Automotive Engineers(SAE) has established standards forspecific analysis of steels. In the 10XXseries, the first digit indicates a plaincarbon steel. The second digit indicatesa modification in the alloys. 10XX meansthat it is a plain carbon steel where thesecond digit (zero ) indicates that thereis no modification in the alloys. The lasttwo digits denote the carbon content inpoints. For example SAE 1040 is acarbon steel where 40 points represent0.40 % Carbon content. Alloy steels areindicated by 2XXX, 3XXX, 4XXX, etc..
10XXPlane Carbon Modification Carbon ContentsSteel in the Alloys In the Points
SAE - AISI Number Classification 1XXX Carbon steels Low carbon steels: 0 to 0.25 % C Medium carbon steels: 0.25 to 0.55 % C High carbon steels: Above 0.55 % Carbon 2XXX Nickel steels 5 % Nickel increases the tensile strength without reducing ductility. 8 to 12 % Nickel increases the resistance to low temperature impact 15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic properties. (Alnicometals) 25 to 35 % Nickel create resistance to corrosion at elevated temperatures.
NICKEL-CHROMIUM STEELS3XXX THESE STEELS ARE TOUGH AND DUCTILE AND EXHIBIT HIGH WEAR RESISTANCE, hardenability and high resistance to corrosion. MOLYBDENUM STEELS4XXX Molybdenum is a strong carbide former. It has a strong effect on hardenability and high temperature hardness. Molybdenum also increases the tensile strength of low carbon steels.
Generally, carbon is the mostimportant commercial steel alloy. Increasingcarbon content increases hardness andstrength and improves hardenability. Butcarbon also increases brittleness and reducesweldability because of its tendency to formmartensite. This means carbon content can be botha blessing and a curse when it comes tocommercial steel.
Most commercial steels are classified into one of three groups: Plain carbon steels Low-alloy steels High-alloy steels
These steels usually are iron with lessthan 1 percent carbon, plus small amounts ofmanganese, phosphorus, sulfur, and silicon. The weldability and other characteristicsof these steels are primarily a product ofcarbon content, although the alloying andresidual elements do have a minor influence.
Low-carbon steels called mild steels, low-carbon steels have less than 0.30 percentcarbon and are the most commonly usedgrades. They machine and weld nicely and aremore ductile than higher-carbon steels. Medium-carbon steels have from 0.30 to0.45 percent carbon. Increased carbon meansincreased hardness and tensile strength,decreased ductility, and more difficultmachining.
High Plane Carbon Steel With 0.45 to0.75 percent carbon, these steels canbe challenging to weld. Preheating,postheating (to control cooling rate),and sometimes even heating duringwelding become necessary to produceacceptable welds and to control themechanical properties of the steelafter welding.
With up to 1.50 percent carboncontent, very high-carbon steels areused for hard steel products such asmetal cutting tools and truck springs.Like high-carbon steels, they requireheat treating before, during, and afterwelding to maintain their mechanicalproperties.
When these steels are designedfor welded applications, their carboncontent is usually below 0.25 percentand often below 0.15 percent. Typicalalloys include nickel, chromium,molybdenum, manganese, and silicon,which add strength at roomtemperatures and increase low-temperature notch toughness.
These alloys can, in the rightcombination, improve corrosionresistance and influence the steelsresponse to heat treatment. But thealloys added can also negativelyinfluence crack susceptibility, so its agood idea to use low-hydrogenwelding processes with them.Preheating might also provenecessary. This can be determined byusing the carbon equivalent formula,which well cover in a later issue.
For the most part, were talkingabout stainless steel here, the mostimportant commercial high-alloysteel. Stainless steels are at least 12percent chromium and many havehigh nickel contents.
Austenitic stainless steels offerexcellent weldability, but austeniteisnt stable at room temperature. Consequently, specific alloysmust be added to stabilize austenite.The most important austenitestabilizer is nickel, and others includecarbon, manganese, and nitrogen.
Ferritic Ferritic stainless steels have 12 to 27percent chromium with small amounts ofaustenite-forming alloys. MartensiticMartensitic stainless steels make up thecutlery grades. They have the leastamount of chromium, offer highhardenability, and require both pre- andpostheating when welding to preventcracking in the heat-affected zone (HAZ).