Effects of chemical elements to steel

Studying the effects of chemical or alloying elements on carbon steel is important not just in determining the proper application of these elements but also in welding or joining of base metals, especially carbon steel, these alloying elements have an effect on the properties of steel during elevated temperatures of welding.

The steel-making process involves a furnace that melts scrap metal and/or iron ore. During this process, elements such as silicon, manganese, carbon, and other elements are added, but what are the effects of these elements on the properties of steel?

The following elements are added to iron and their effects will be discussed in the next sections.

Carbon - this element is considered the most important alloying element in steel. It typically exists from less than 0.5% up to 2%. However, almost all weldable steels have less than 0.5% carbon content. Increasing the carbon content also increases the hardness and tensile strength but sacrifices weldability.

Two major effects of raising carbon that are seen to be detrimental are:

• Raises the ductile-to-brittle transition temperature - this just means that steel becomes more brittle at higher temperatures than normally in colder temperatures. This is especially important when welding in colder climates.
• Lowers the maximum impact energy - since carbon increases the hardness, it lowers the maximum impact that the steel can handle at a lower carbon content.

Manganese - steels usually contains at a minimum 0.3% manganese. It assists in the deoxidization of steel, prevents the formation of iron sulfide inclusions and promotes greater strength by increasing hardenability and increases notch toughness. Some low carbon steels reach 1.6% but beyond this retained austenite can be encountered which can lower the impact properties of carbon steel for each 0.1% increase. The effect of manganese in terms of tensile properties is about one-quarter that of carbon.

Chromium - increases the hardenability of steel and improves the corrosion resistance of alloys in oxidizing medium. Stainless steels contain chromium in amounts of at least 12%.

Nickel - increases hardenability and improves toughness and ductility at the same time. It improves the response of steel to thermal treatment and improves steel's toughness at low temperatures. It also improves corrosion resistance.

Molybdenum - acts like carbon and raises the ductile-to-brittle transition to higher temperatures. It is often added to steel to improve its elevated temperature properties and resistance against hydrogen.

Silicon - deoxidizes steel and improves notch toughness. Oxygen affects notch toughness of steel and in order for steel to have improved notch toughness it must be "fully killed". Fully killed is defined as any steel with minimum residual silicon content of 0.10%.

Aluminum - added to steel in small amounts as a grain refiner for improved toughness. Grain refinement prevents solidification cracking in welding and thus prevent the occurrence of defects. Aluminum also acts as deoxidizer.

Vanadium - increases the hardenability of steel. It is very good at this purpose that amounts greater than 0.05% will increase the tendency of steel to become embrittled during thermal stress relief.

Niobium - similar to vanadium, it increases the hardenability of steel but with its strong affinity for carbon, it can combine with carbon in steel and thus result in an overall decrease in hardness. This element is normally added to austenitic stainless steel as a stabilizer improving its welding properties.

Elements with detrimental effects

Phosphorus - raises the transition temperature by 7 degrees C for each 0.010%

Sulfur - these elements exist in steel as sulfide inclusions which cause brittleness and decrease in toughness.

Dissolved gases such as hydrogen, oxygen and nitrogen dissolve in molten steel and can cause embrittlement. Steel refining process called vacuum degassing removes these elements down to less than 0.01%. Shielding gas also prevents these gases from going into the molten metal.

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