The Role Of Elements In Castings And The Order Of Addition

In the casting production process, in order to adjust the chemical elements to the required range, we need to add alloying elements. The amount of each element added to the casting, the time of addition and the order of addition will affect the quality of the casting. We try to analyze several commonly used elements:

I. The role of each element and the principle of addition

(1)Carbon (C)

Function:

Matrix strengthening: C is the core solid solution strengthening element of steel, forming cementite (Fe₃C) with iron to improve hardness and strength.

Solidification control: High C content will reduce the fluidity of molten steel and increase the tendency of shrinkage.

Addition principle: The content needs to be adjusted according to the target performance (generally controlled at 0.15%~0.3% in low alloy steel).

Over-risk: When C＞0.5%, the toughness decreases significantly and the weldability deteriorates.

(2) Silicon (Si)

Function:

Deoxidizer: Preferentially reacts with O to form SiO₂ to purify the molten steel.

Solid solution strengthening: dissolves in ferrite to increase strength (tensile strength increases by about 4 MPa for every 0.1% Si increase).

Addition principle: add in the late stage of smelting (reduction period) to avoid oxidation loss (such as ferrosilicon alloy).

Risk of excessive content: the content is controlled at 0.2%~0.5%, too high will reduce toughness.

(3) Manganese (Mn)

Function:

Deoxidation and desulfurization: generate MnO (deoxidation) with O, and generate MnS (desulfurization) with S.

Improve hardenability: delay pearlite transformation and improve martensite hardenability.

Addition principle: add in batches during the oxidation period (deoxidation + desulfurization), and add in the reduction period (if burned).

Risk of excessive content: the content is controlled at 0.8%~1.5%, too high will easily lead to temper brittleness.

(4)Phosphorus (P)

Function:

Harmful elements: solid dissolve in ferrite, reduce plasticity and toughness (cold brittle tendency).

Solid solution strengthening: trace amounts of P can improve strength, but the amount needs to be strictly controlled. It is not recommended to add it in medium frequency furnace production.

Control principle: Try to choose low-phosphorus raw materials (such as scrap steel) and avoid additional addition during smelting.

Risk of excessive amount: the content needs to be less than 0.035% (high-quality steel requires less than 0.025%).

(5)Sulfur (S)

Function:

Harmful elements: generate FeS with Fe, causing hot brittleness (high temperature processing cracking).

Inclusion control: needs to be combined with Mn to generate MnS (reduce harm).

Control principle: desulfurization by adding Mn (Mn:S ratio recommended 2:1~3:1).

Risk of excessive amount: the content needs to be less than 0.035% (special steel <0.02%).

(6)Chromium (Cr)

Function:

Improve hardenability: delay austenite decomposition and increase martensite hardness.

Corrosion resistance: form a dense Cr₂O₃ oxide film (such as stainless steel).

Refine grains: inhibit austenite grain growth.

Addition principle: add during the melting period (high melting point, high temperature dissolution required). The content is usually 0.5%~2.0% (adjusted according to corrosion resistance or wear resistance requirements).

(7)Molybdenum (Mo)

Function:

Refine grains: inhibit austenite grain coarsening and improve toughness.

High temperature stability: improve red hardness and creep resistance.

Solid solution strengthening: enhance matrix strength.

Addition principle: add during the melting period (similar to Cr) to avoid high temperature volatilization. The content is usually 0.1%~0.3% (higher for high molybdenum steel).

Ⅱ. Interaction between elements

(1)Synergistic effect of C and Si/Mn

Deoxidation balance: Si deoxidizes first, Mn assists in desulfurization, but excessive Si will inhibit the desulfurization effect of Mn.

Phase transformation effect: When the C content is high, Mn may delay the pearlite transformation, resulting in an increase in residual austenite.

(2)Complementary effect of Cr and Mo

Hardenability superposition: Cr and Mo jointly improve hardenability, suitable for high-strength steel (such as HSLA).

Corrosion resistance synergy: Cr provides a passivation film, and Mo enhances pitting resistance (such as Cr-Mo combination in stainless steel).

(3)Synergistic harm of P and S

Low-temperature brittleness: P aggravates cold brittleness, and S causes hot brittleness. The risk needs to be reduced through Mn and process control.

Ⅲ. Adaptability of medium-frequency furnace smelting process

(1)Optimization of addition sequence

Melt period: Add high-melting-point elements such as Cr and Mo to ensure full dissolution.

Oxidation period: Add Mn in batches (deoxidation + desulfurization). Products with high requirements can use oxygen blowing technology, but the amount of oxygen blowing needs to be controlled to avoid excessive oxidation.

Reduction period: Add Si (final deoxidation) and supplement Mn (if burned).