Niobium is the alloying elements of narrow ³-Fe phase, has a certain solubility in the ±-Fe. Containing 0.5% Nb in iron-base alloy is sufficient to achieve the effect of dispersion strengthened.
Niobium is also a strong carbide forming elements, niobium carbide is very stable at high temperature, and only slightly less than the titanium carbide. Due to the niobium with good heat-resistance, so it has a wide range of applications in low-alloy heat-resistant steel and high-alloy heat-resistant steel. Multicomponent alloys niobium pearlitic low-alloy heat-resistant steel use in the steam generator, the temperature can up to 550 ° C. Niobium applied to heat-resisting steel which containing 12% chromium, improve the hot strength of steel. in high-alloy heat-resistant steel, niobium carbide precipitation occupies an important position in its thermal, therefore there is a certain ratio between the carbon content and niobium content in the steel. Low carbon (0.04% to 0.07%) heat-resistant austenitic steel, the niobium content should be 10x% C. Therefore, the niobium content in the heat-resistant high-alloy steel is generally 1% to 2%.
Tantalum and niobium in the periodic table is in the same family, the role of tantalum in heat-resistant steel is similar with niobium. Tantalum is expensive, in the ordinary heat-resistant steel is not add tantalum substantially, but usually brought into with the addition of niobium in the steel. Usually add tantalum only in casting high temperature alloys.
Boron and nitrogen have a strong affinity with oxygen. The trace boron powder (0.001%) in steel can increase its hardenability member. Boron adsorption on the austenite grain boundaries, reducing the intergranular energy, impede the nuclei of ferrite formation, thus prolonging the incubation period of proeutectoid ferrite and bainite transformation, boron only in solid soluble form present in the steel in order to play an effective role. When boron react with residual nitrogen and oxygen in the steel and form a stable inclusions, will have an adverse effect on the performance of the steel. In pearlitic heat resistant steel, trace boron can improve the high temperature strength of the steel; added 0.025% boron in the austenitic heat resistant steel can be improved its creep resistance, but the boron content is higher, contrary to its role. Add boron to strengthen grain boundary and endurance strength is very important for enhancing the heat-resistant steel. The boron atom is mainly distributed in the grain boundary, and therefore boron plays an important role on the grain boundary strengthening.
Rare earth elements to improve the oxidation resistance of the heat-resistant system have obvious role. An oxide of the rare earth elements can increase the adhesion between the matrix metal and the oxide film, because of rare earth oxides on the base metal have the "pinning" effect. According to metal powder supplier, rare earth metal lanthanum and cerium can reduce the volatility of Cr 2 O 3 , improve the composition of the oxide, becomes more stable (Cr La), 2 O 3 oxide film, lanthanum within the range of 1100 ~ 1200 is easily to decompose NiO film. REE is also a good desulfurization degassing agent in steel, can clear the state of the shape and distribution of the inclusions and other harmful impurities (such as arsenic, antimony, bismuth, etc.) can be improved, so as to improve and enhance the quality of steel and heat properties. Rare earth elements have a certain role in refining the grain size of the steel. The rare-earth elements have very strong affinity with oxygen, sulfur, phosphorus, nitrogen, hydrogen and the like, with arsenic, antimony, lead, bismuth, tin and so on also can form a high melting point compound. So it is a good deoxidation, get rid of sulfur and additive to remove other unwanted impurities and gases. REE can improve the creep resistance of the heat-resistant steel. Added mixed rare earth in Cr 25 Ni 20 heat-resistant steel to can 25% improve its lasting strength at 650 ; in Cr 13 Ni 16 Nb and N, 36CrTiAl steel added 0.01% Ce, for its creep strength have varying degrees improving.
Source:http://www.mhcmp.com
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