X8CrNb 18

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The kinds of stainless steel

1, ferritic stainless steel: including chromium 12% ~ 30%. The corrosion resistance and toughness and weldability with containing cr increases with the increasing of quantity, chloride stress corrosion resistance performance is better than other types of stainless steel. Belong to this category of Crl7, Cr17Mo2Ti, have Cr25, Cr25Mo3Ti, Cr28, etc. Ferritic stainless steel because contain chromium volume is high, corrosion resistance and oxidation resistance are relatively good, but the mechanical properties and the process performance is poorer, more for stress of acid structure and as little antioxidant use steel. This kind of steel can resist the atmosphere, nitric acid and brine solution of the corrosion, and has high temperature oxidation resistance, good thermal expansion coefficient characteristics such as small, used to nitric acid and food factory equipment, also can make in the high temperature parts, such as gas turbine parts, etc.

2, austenitic stainless steel: including chromium greater than 18%, still contain 8% of nickel and a few molybdenum, titanium, nitrogen element. Comprehensive performance is good, resistant to a variety of medium corrosion. Austenitic stainless steel grades of commonly used one Cr18Ni9, 0 Cr19Ni9, etc. 0 Cr19Ni9 steel the wC < 0.08%, Number marked as "0". This kind of steel contain a lot of Ni and Cr, make steel at room temperature is austenitic state. This kind of steel has good plasticity and toughness, weldability and corrosion resistance, in oxidizing and reducing medium corrosion resistance were well, used to make acid equipment, such as corrosion resistant containers and equipment, pipeline, resistant to the lining of the equipment parts nitric acid. Austenitic stainless steel generally USES solid solution processing, steel is heated to a 1050 ~ 1150 ℃, and water, for single-phase austenitic organization.

3, austenitic-ferritic duplex stainless steel: with austenitic stainless steel and the advantages of ferrite, and has the plastic. Austenitic and ferritic organization about half of the stainless steel. Containing C in the situation of low, Cr content at 18% ~ 28%, 3% ~ 10% in Ni content. Some steel also contain Mo, Cu, Si, Nb, Ti, N, alloy element. This kind of steel with austenitic stainless steel and the characteristics of ferrite, compared with ferrite, plastic, higher toughness, no room temperature brittleness, intergranular corrosion resistant performance and welding properties are improved significantly and also maintain a ferritic stainless steel 475 ℃ brittle and high thermal conductivity, and has the characteristics such as plastic. Compared with austenitic stainless steel, high strength and corrosion resistance and intergranular corrosion resistant to chloride stress have increased significantly. Duplex stainless steel has excellent resistance to corrosion resistance hole, is also a kind of festival nickel stainless steel.

4, martensite stainless steel, high strength, but plastic and solderability is poorer. Martensite stainless steel grades of commonly used one Cr13, 3 Cr13 and so on, because it contains carbon is higher, it has high strength, hardness and wear resistance, corrosion resistance is a bit poor, mechanical performance requirements for higher, corrosion resistance performance of some parts of the general requirements, such as spring, turbine blade, hydraulic press valves, etc. This kind of steel is quenching and tempering treatment in use.

(a) strength, tensile strength, yield strength)

The strength of the stainless steel by all sorts of factors is uncertain, but the most important and the most basic factors is one of the different chemical factors add, mainly is the metal elements. Different types of stainless steel due to the difference of the chemical components, the different characteristics of strength.

(1) markov shape stainless steel

Markov shape with ordinary stainless steel Alloy steel has the same through the quenching realize hardening of the characteristics, so it can choose brand and heat treatment conditions through to get wider range of different mechanical properties.

Markov shape from the aspects of stainless steel to distinguish, belongs to the iron-chromium-carbon is stainless steel. And then can be divided into martensite chromium martensitic stainless steel and chromium in ni-based stainless steel. Chromium martensitic stainless steel is in adding chrome, carbon and molybdenum elements such as a change in the level of trend and in martensite stainless steel is adding nickel chrome of strength characteristics are described below.

Chromium martensitic stainless steel is the hardening-tempering conditions, increase the content of chromium can make ferritic content increased and will reduce hardness and tensile strength. Low carbon chromium martensitic stainless steel in annealing conditions, when chrome content increases hardness raised, and elongation rate declined slightly. Chrome content in certain conditions, the increase of the content of carbon in the steel hardened hardness also will increase, and the plastic reduced. Add the main purpose is to improve the molybdenum strength of steel, hardness and secondary sclerosis effect. In the low temperature quenching, molybdenum add efficiency were compared. Content usually less than 1%.

In martensite chromium stainless steel in ni-based, containing a certain amount of nickel can reduce the steel δ ferritic content, make steel get the maximum hardness value.

Markov shape the chemical composition of stainless steel is characterized in 0.1% 1.0% C, 12%-27% of the different components Cr based on tungsten, molybdenum, add vanadium, and niobium elements. Due to the structure of the organization is body heart cubic structure, thus in the high temperature strength fell sharply. And in 600 ℃ below, the high temperature strength in all kinds of stainless steel in the highest, and the creep strength is the highest.

(2) iron grain size stainless steel

According to the results, when chrome content is lower than 25% when the ferrite organization would inhibit the formation of martensite organization, and with the increase of the content of chromium its strength down; More than 25% of the alloy due to solid solution strengthening function, strength slightly increased. The increase of the content of molybdenum can make it easier to get the ferrite organization, can promote α 'phase, б and the precipitation in x, and approved by solid solution strengthening its strength after improvement. But at the same time also improves the notch sensitivity, and reduce the toughness. Mo iron body stainless steel, improve the intensity of the role is greater than the role of chrome.

Iron grain size characteristics of the chemical composition of the stainless steel is to contain 11%-30% Cr, of which add niobium and titanium. The high temperature strength in all kinds of stainless steel is the lowest, but the strongest resistance to thermal fatigue.

(3) austenitic stainless steel body

Austenitic stainless steel body more carbon content, due to its solid solution strengthening action to make strength improved.

Austenitic stainless steel body of chemical composition characteristics based on chromium, nickel, tungsten, molybdenum added element niobium and titanium. Because of its organization for surface heart cubic structure, thus in the high temperature high strength and creep strength. Also because of steels are big, so than iron grain size stainless steel thermal fatigue strength is poor.

(4) duplex stainless steel

On chrome content is about 25% of the mechanical properties of the duplex stainless steel research indicates that, in α + r duplex zone nickel content increases r phase also increases. When the steel chrome content was 5%, the yield reached a high strength steel; When nickel content is 10%, the strength of steel to achieve maximum.

(2) creep strength

Because the role of external force over time with the increase of the distortion phenomenon called creep. In certain temperatures especially under high temperature, the larger the load is creep and the faster; In certain load, the higher the temperature is, and the more time, the greater the chance of creep. And in contrast, temperature is lower creep speed is more slowly, in low to certain temperature creep is out of the question. The lowest temperature varies according to the steel is, in general, pure iron in 330 ℃ or so, and while for stainless steel has taken various measures to strengthen the, so the temperature is above 550 ℃.

And other steel, melting way and oxidize and methods, coagulation treatment, heat treatment and processing the creep characteristics of stainless steel has very big effect. According to introducing, in the United States to 18-8 stainless steel to creep strength test showed that the same parts of the same ingot from try material of creep rupture of the standard deviation is this time average of about 11%, and different from the ingot, in different parts of the standard deviation and try material is the average of two times. And according to in Germany on test results show that the five times in 10 power h time 0 Cr18Ni11Nb strength of steel for less than 49 MPa to 118 MPa, sent a scattered.

(3) the fatigue strength

High temperature fatigue is refers to the material under high temperature cycle because changes by the process of stress injuries to fracture process. The research results show that, in a high temperature, 10 eight times power times fatigue strength is the high temperature high temperature tensile strength of 1/2.

Thermal fatigue is to point to in the heat (inflation) and cooling (contract) is the process of, when the temperature change and suffer from external binding, in material corresponding to its own internal expansion and contraction deformation produced stress, and make the material damage occurred. When fast heating and cooling repeatedly when the stress will have impact, the produced stress and often more than, at this time some material is brittle failure. This phenomenon is called Zhi impact. Thermal fatigue and thermal shock is a similar phenomenon, but the former mainly with large plastic strain, and the latter mainly is the destruction of the brittle failure.

The composition and stainless steel heat treatment temperature conditions on the fatigue strength have influence. Especially when the carbon content of high temperature increases obviously improve the fatigue strength, solid solution heat treatment temperature also have significant influence. Generally speaking, iron body good stainless steel thermal fatigue performance. In the austenitic stainless steel, high silicon and in high temperature has good extensibility grades have good thermal fatigue performance.

The smaller the thermal expansion coefficient, in the same heat cycle effect should be under the smaller variables, the smaller the deformation resistance and breaking strength, the higher the longer service life. Can say Cr17 1 of markov shape stainless steel fatigue life expectancy, and 0, 0 Cr19Ni9 Cr23Ni13 and 2 Cr25Ni20 austenitic stainless steel body such as the fatigue life of the shortest. Another is more likely to occur due to the casting forging thermal fatigue cause damage. In room temperature, seven of the 10 power times fatigue strength is 1/2 of the tensile strength. And high temperature than the fatigue strength, it is known that the temperature of the room temperature to from temperature range fatigue strength is no difference.

(4) impact toughness

Material in impact load, load deformation curves of the area including called impact toughness. For the martensite stainless steel casting time, when nickel content was 5% the impact toughness is low. Along with the increase of the content of nickel, steel strength and toughness can be improved, but nickel content is more than 8%, strength and toughness values and a decline. In martensite stainless steel is adding chrome nickel molybdenum, can improve the strength of steel and can keep the toughness unchanged.

Grain size in iron content of molybdenum in stainless steel increased although can improve the intensity, but the gap is also improve the sensitivity and toughness decline.

Austenitic stainless steel in shape with stable austenitic organization and chromium ni-based austenitic stainless steel toughness (room temperature toughness and low temperature toughness) is very good, and used in in room temperature under low temperature and the various environmental in use. To have a stable austenitic organization and chromium manganese department austenitic stainless steel. Add nickel can further improve the toughness.

Duplex stainless steel with the impact toughness of the nickel content increases. Generally speaking, in a + r two phase zone its impact toughness stability in the 160-200 J range.