The Effect of Common Elements on Cast Steel

Adding C to the steel casting process is a common way we cast. When the C content is less than 2%, we call it cast steel; when it is higher than 2%, we call it C steel. However, C itself does not have strength and hardness. However, as Fe(3)C, the iron carbide in the solid solution, C is the primary controlling element for strength and hardness.

The leading role of C:

In stainless steel casting, as the C content increases, it increases the strength, hardness, and hardenability of the steel. However, it reduces plasticity, toughness, and magnetic and electrical properties. The combination of C and certain alloying elements in steel forms various carbides, which have different effects on the properties of steel.
C content in some steel casting ranges: C steel 0.03 ~ 1.04%, high-speed tool steel 0.75 ~ 1.60%, hot work tool steel 0.22 ~ 0.70%, cold work tool steel 0.45 ~ 2.85%.

We see Mn in molten steel in the deoxidation and desalinization process, but at this point, it remains in steel in amounts less than 1%. When the quantity of Mn in steel exceeds 1%, Mn is an intentionally added alloying element.

The main role of Mn:

To improve the tensile strength, hardenability, high toughness, and processing properties of steel casting. In steels containing S, Mn minimizes the hot and cold embrittlement caused by S. Steel with high Mn content has high wear resistance after hard work or impact. But there is a tendency to promote the grain growth of steel and increase the second type of tempering brittleness. The element of Mn is used in structural steel, steel reinforcement steel, and spring steel.
Mn in some steel casting in the content range: C steel 0.25 ~ 0.65%, Mn steel 1.6 ~ 1.9%, cold tool steel 0.30 ~ 2.50%, authentic Cr – Ni stainless steel 2.00 ~ 15.5%.

Si is a ferrite-forming element. It raises the A(1) and A(3) temperatures. Since Si has a graphitic effect, it is generally combined with Mn in steel as a stabilizer of carbides. It is a commonly used deoxidize.

The main role of Si:

In electrical sheet steels, Si increases permeability and resistivity and allows shallow hysteresis losses. Si makes some high-temperature resistant steels resistant to oxidation. And steel casting improves hardenability, strength, and impact toughness through the combination of Si and Mn. Especially after quenching and tempering, it can improve the yield limit and elastic limit of steel. The high Si content of steel casting, its magnetic properties, and resistance are significantly increased. However, Si tends to promote graphitization, and when steel contains high levels of C, the effect is even greater.
The content of Si in ordinary steel ranges from 0.17 to 0.37%.

The main role of Cr:

Stainless steel casting contains enough C when Cr improves the hardness of the steel. Generally, low Cr steels containing 1% C are very hard. Cr added to low C steel can improve strength but reduce ductility. Cr improves the high-temperature strength and hardenability of steel; in high C steel and wear resistance.

When added in large amounts, up to 25%, Cr improves corrosion resistance due to forming a protective oxide layer on the surface of the steel. In combination with elements such as Ni, it improves oxidation resistance and the thermal strength of the steel and further improves corrosion resistance. Cr promotes grain growth, leading to increased brittleness of steel. Cr does widely use element in the structural, tool, bearing, stainless, and heat-resistant steel.

W is a solid carbide-forming element. It forms the very hard and stable carbides W(2)C, WC, and the complex carbide Fe(4)W(2)C. These carbides dissolve very slowly and only at very high temperatures. W is an important component of tool steels, especially high-speed tool steels. In these steels, W significantly increases the hardness after secondary hardening. As a ferrite-forming element, W lowers the A(4) temperature and raises the A(3) temperature.

The leading role of W:

W inhibits grain growth and therefore has a grain refining effect. W reduces decarburization during hot working and heat treatment. It increases the high-temperature hardness of quenched and tempered steels. In some high-temperature alloys, W increases creep strength. The stainless steel casting containing W is heated to 600~700℃ carbide still does not precipitate, thus avoiding the softening of the steel. It is one of the elements used more in high-speed tool steel and alloy tool steel. However, it is not suitable for cast steel oxidation resistance.
W in some steel content range: W-Cr steel 1.75%, high-speed tool steel 1.15 ~ 21.0%, hot work tool steel 0 ~ 19.0%, cold work tool steel 0 ~ 2.0%, impact-resistant steel 0 ~ 3.0%.

Mo is a robust carbide-forming element. It forms the rigid and stable carbide Mo(2)C, as well as other complex carbides such as Fe(4)Mo(2)C and Fe(21)MoC(6).

The leading role of Mo:

A small amount can effectively reduce the transformation rate and improve cast steel’s high-temperature strength and creep resistance. It can also enhance the corrosion resistance of stainless steel, especially in hydrochloric acid solutions. W has a similar role in high-speed tool steel, often Mo instead of W. This reduces carbide build-up in steels containing W and improves mechanical properties. Mo is one of the essential components in Mo steels, which have high tensile strength and good plasticity. In Cr steels, Mo is added to improve machinability and mechanical properties. In Ni-Mo steels, Mo increases the surface hardness of cast stainless steel.
Mo in some steel content range: Mo steel 0.15 ~ 0.60%, high-speed tool steel 0 ~ 10.0%, hot work tool steel 0 ~ 5.5%, cold work tool steel 0 ~ 1.8%, authentic Cr – Ni stainless steel 0.~ 4.0%, martensite Cr steel 0 ~ 1.25%, ferrochrome steel 0 ~ 1.25%.

V is a carbide-forming solid element. It forms carbide VC. V is present in cast steel as fine-grained and diffuse carbides and nitrides. They do not decompose at average heat treatment temperatures and can inhibit grain growth. A sufficient amount of carbon and soluble V at elevated temperatures causes the most significant secondary hardening effect observable.

The leading role of V:

V is an essential grain refining element. It inhibits austenite growth, and as little as 0.1% V will effectively inhibit grain growth during hardening. However, grain growth starts immediately when cast steel is heated to a temperature where grain growth inhibits the dissolution of carbide and nitride particles.
Iron, carbon content ratio greater than 5.7 can prevent or reduce the media on the stainless acid steel intergranular corrosion. And significantly improve the cast stainless steel resistance to high temperature, high pressure, and hydrogen corrosion ability. Can refine the grain and slow down the transfer rate of alloying elements, but the cast iron steel’s high-temperature oxidation resistance is not good.
V in some steel and alloy content range: chromium – V steel 0.10 ~ 0.20%, high-speed tool steel 0.90 ~ 5.25%, hot tool steel 0 ~ 2.20%, cold tool steel 0 ~ 5.15%

Co is a carbide-forming element. It has a slightly stronger tendency to form carbides than iron. It improves the high-temperature properties of steel and its resistance to oxidation and corrosion. Co is a crucial component of some super-hard high-speed tool steels, some permanent magnet alloys, and high-temperature alloys.

The leading role of Co:

Co substantially increases strength and toughness by promoting the precipitation hardening process. Co dissolves into ferrite or austinite and inhibits the softening during temperature rise.
The range of Co content in some steels and alloys: 0~13.0% for high-speed tool steels, 0~4.5% for hot work tool steels

Ti is a solid carbide-forming element. In the solid solution state, the reliable solution strengthening effect is extremely strong but simultaneously reduces the toughness of the solid solution.

The leading role of Ti:

In medium chromium steels, Ti pulls back carbon from the solid solution and reduces martensite hardness and hardenability. In high chromium steels, Ti prevents the formation of austenite. In austenitic stainless steels, Ti pulls back carbon from the solid solution at high temperatures, thus preventing chromium carbides from forming at grain boundaries. In austenitic high-temperature alloys, Ti promotes precipitation hardening. In sedimentary high-strength low-alloy steels, Ti improves the toughness of the steel. Ti can refine the grain and fix the carbon, benefiting cast iron steel’s weldability.
The content of Ti in high strength low alloy steel ranges from 0~0.10%

Lead has limited solubility in molten steel. Since it has a density much more excellent than steel, it sinks in the molten steel and separates from the steel. It is present in the steel as inclusions. These inclusions are soft and act as interstitial lubricants. Its content of about 0.20% and the presence of very small particles can improve the machinability of the steel without significantly affecting other properties.

Ni does not form carbides. The presence of Ni in cast iron steel destabilizes the carbides of iron and therefore promotes graphitization. The Ni acts to strengthen ferrite by developing simple replacement solid solutions. As an austenite-forming element, Ni stabilizes austenite by raising the A(4) temperature and lowering the A(3) temperature. If the pure iron contains more than 25% Ni, the resulting alloy is pure austenite even if it is slowly cooled to room temperature.

The leading role of Ni:

In alloy steels containing up to 5% Ni, Ni improves strength and toughness. Ni enables austenite formation with a high chromium content, giving austenitic chromium-Ni stainless steels. It is widely used, especially in stainless steel and heat-resistant steel. High Ni steels containing small amounts of carbon significantly increase the thermal hysteresis of the isotropic heterocrystalline transformation. This leads to the retention of martensite in martensitic aged steels when heated to 600°C. Ni reduces the coefficient of thermal expansion, and in high Ni alloys, it increases the permeability. When the content of Ni in cast steel is high, combining it with chromium can significantly improve the corrosion and heat resistance of the steel.

Nb increases strength by precipitation hardening and further refining the ferrite grain.

The main role of Nb:

Small amounts of Nb (approx. 0.02%) can effectively increase carbon steel’s tensile and yield strength. Nb is an essential component of some high-strength low-alloy steels.
When the content exceeds 8 times the carbon content, it can fix almost all the carbon in cast steel. Autentic steels prevent intergranular corrosion of cast steel by oxidizing media. Due to the fixation of carbon in steel and precipitation hardening effect, it can improve the high-temperature properties of hot-strength steels, such as creep strength.

Copper is an austinite-forming element. Since copper tends to graphitize, it is only added to mild steel and in quantities no greater than 1.5%.
The main role of copper is to improve corrosion resistance and increase the alloy’s tensile strength by means of precipitation hardening. Copper can slightly increase the yield strength in those steels where precipitation hardening does not occur. However, the plasticity and toughness of stainless steel casting are reduced. Carbon steel increases hardenability and decreases ductility. When the copper content exceeds 0.4%-0.5%, it makes the surface of steel parts easy to produce cracks during hot working.

Al has a lower carbide formation capacity than iron. Al promotes graphitization in stainless steel casting.

The leading role of Al:

Al and nitrogen or O to produce effective fine dispersions to inhibit grain growth. It generate an effective surface hardening layer through the lower temperature diffusion of nitrogen (nitriding). And Al makes steel corrosion resistant by developing a solid layer of Al oxide on the surface of cast iron steel. Al refines cast stainless steel grains, thereby increasing the strength and toughness of the steel.
The content of Al in some steels and alloys ranges from 0.01 to 0.03% in ordinary steel, 0 to 0.30% in ferritic stainless steel, and 0 to 1.5% in precipitation hardening stainless steel.

B is a very hard solid that melts at 2300°C.

The leading role of B:

Adding trace amounts of B (0.0005 to 0.005%) in fully deoxidized cast stainless steel can significantly improve the hardenability of the steel. This is due to a reduction in the phase transition rate during cooling, but when the carbon content increases, the hardenability decreases. Therefore, B added to low or medium carbon steel with <0.6% carbon content has a significant effect. If you add a small amount of B, reduce the amount of other more expensive alloying elements by half, still casting steel to maintain the same transformation rate. B improves the ductility and mechanical properties of cast stainless steel.

S is the element that is most harmful to cast steel. It has the most detrimental effect on steel by forming the brittle sulfide FeS. Sulfide is present in steel in quantities as low as 0.01% and can also excite sulfide precipitation along grain boundaries.

The main role of S:

When a sufficient amount of manganese is added to the cast steel, S forms MnS, which is plastic at hot working temperatures. S can increase the non-metallic inclusions in the steel so that the steel strength is reduced and significantly reduces the steel welding properties. In hot work, easy to produce brittle (hot brittle), but a slightly higher S content can improve the machinability of low carbon steel.
The content of S in ordinary steel is ≤ 0.045%.

S is the element that is most harmful to cast steel. S has the most detrimental effect on steel by forming the brittle sulfide FeS. Sulfide is present in steel in quantities as low as 0.01% and can also excite sulfide precipitation along grain boundaries.

The main role of S:

When a sufficient amount of manganese is added to the cast steel, S forms MnS, which is plastic at hot working temperatures. S can increase the non-metallic inclusions in the steel so that the steel strength is reduced and significantly reduces the steel welding properties. In hot work, easy to produce brittle (hot brittle), but a slightly higher S content can improve the machinability of low carbon steel.
The content of S in ordinary steel is ≤ 0.045%.

N in steel can both form nitrides and dissolve in the interstices after solidification. In both cases, N embrittles the steel and makes it unsuitable for cold working. Therefore, the N content must be as low as 0.002% to produce soft steels with high plasticity. Reducing the N content also reduces quench aging. In hot rolled or forged steels, phosphorus makes so-called “anomalous zones,” which become weak spots in the steel.

Hydrogen can diffuse out in steel at room temperature. This diffusion is more effective during a slow temperature rise. A hydrogen content greater than about 0.0005% leads to a reduction in the ductility of the steel. Hydrogen content above 0.0005% can cause internal rupture of the steel. This generally occurs during the cooling of the metal after rolling or forging.
It makes the steel susceptible to impermissible defects such as white spots, which can lead to hydrogen embrittlement and is also an essential factor leading to cold cracking (delayed cracking) in the heat-affected zone of the weld.

During the blowing process of steelmaking, a large amount of O is blown into the melt pool. By the end of the blowing process, the steel contains excess O. If not deoxidized, it will affect the subsequent pouring operation. And in the steel solidification process, O is in the form of oxide precipitation in large quantities. Cast stainless steel will also produce oxide non-metallic inclusions, reducing steel’s plasticity and impact toughness, making the steel brittle.

As content does not exceed 0.20%, the general mechanical properties of cast stainless steel are not significant but increase the sensitivity of tempering brittleness.

A small amount of Zr has degassed, purified, and refined the role of grain. The low-temperature toughness of steel is beneficial. It can also eliminate the aging phenomenon and improve the steel stamping properties.