Introduction of correlation between steel material properties and temperature

 There are two types of steel material coefficient related to temperature: one is steel material coefficient related to mechanical properties of steel materials; The other is the material coefficient of steel related to heat conduction. The former are E, G, v, a; Among the latter are C(specific heat capacity), ρ (density), k(heat conduction coefficient) and so on. These coefficients are not actually constant, but vary with temperature. However, when the temperature is not high, the average value is usually treated as a constant, but in the case of high temperature and large change, it must consider its change with the temperature.

Relationship between elastic coefficient and temperature of iron and steel materials

The elastic coefficient E and shear modulus G decrease with increasing temperature, and Poisson's ratio v changes little with temperature. E, G and temperature are measured by static method and dynamic method, the former is tested by loading in a high temperature furnace, the latter is measured by vibration method or ultrasonic pulse method. The vibration method is to make the specimen do elastic vibration in the high temperature furnace, and determine the elastic constant by measuring the frequency. The ultrasonic rule is to give the specimen ultrasonic waves, and measure E, G, v by measuring the propagation speed of the waves.

The relationship between heat coefficient and temperature

The thermal coefficient of metal materials is generally linear with the temperature, the linear expansion coefficient a generally increases with the increase of temperature, the thermal conductivity k decreases with the increase of temperature, and the specific heat capacity increases with the increase of temperature. Through the linear slope or curve of the relationship between the heat coefficient and temperature measured by the test, the change of the heat coefficient of the specific material with the temperature can be known.

Thermal fatigue of steel materials

When the ductile steel material increases with the temperature, even if the stress exceeds the yield point, it will not be immediately damaged, but even if the stress level is low, if there is a large temperature change repeatedly, it will eventually crack due to fatigue and lead to damage. This phenomenon is called thermal fatigue.

Assume that at the beginning of the test, the rod is fixed at the highest temperature and then cooled to produce tensile stress, with OAF being a stress variation line. Then, if the heat is reheated, the stress-strain line begins to move down parallel to OA, yielding at a stress lower than the cooling cycle tension, and finally reaching point E. If it is kept at the highest temperature for a period of time, the compressive stress decreases to the E' point due to stress relaxation. If it begins to cool again, it rises along E 'f ', reaching F' point at the lowest temperature. Since no pressure relaxation occurs at the lowest temperature. If the heating starts again, the line falls along F'E" to E" at the highest temperature. Here, due to stress relaxation, the stress decreases and moves to E"'. If cooling starts again, F" is reached at the lowest temperature along the curve E"'F".

If this cooling-heating cycle is repeated, the stress-strain diagram depicts a hysteresis curve each time, and the associated retroplastic strain is the cause of thermal fatigue. The maximum and minimum temperature, the average temperature, the holding time of the maximum temperature, the repetition rate, the elastoplastic property of the material are all factors that affect the thermal fatigue.

The strength of thermal fatigue refers to the relationship between the plastic strain εP of a cycle and the number of repetitions N to reach failure.

The above mentioned is only the unidirectional thermal stress fatigue of the material, and the thermal fatigue of the actual structure is multi-directional and is a special research field.

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Various structures and inclusions in steel (Chapter Three)

 Standard for determination of non-metallic inclusions in steel

1. Scope

Microscopic evaluation method for nonmetallic inclusions in rolled or forged steels with compression ratio greater than or equal to 3 by standard spectrogram.

2 Sampling

The morphology of inclusions depends to a large extent on the degree of compression deformation of the steel, so it is only possible to compare the measured results on the sections prepared by sample billet after similar degree of deformation.

The polished surface area of the specimen for measuring inclusion content shall be approximately 200mm²(20mm X10mm) and shall be parallel to the longitudinal axis of the steel, located halfway from the outer surface of the steel to the center.

If the product standard does not specify, the sampling method is as follows:

Steel rods or billets with a diameter or side length greater than 40mm: the inspection surface is the partial radial section from the outer surface of the steel to the center.

3. principle

The observed test site was compared with the standard spectrum, and each type of inclusion was rated.

These rated images are equivalent to a square field of view with an area of 0.50mm² on a longitudinally polished plane 100 times lower.

According to the morphology and distribution of inclusions, the standard rating chart is divided into five categories: A, B, C, D and DS.

These five categories of inclusions represent the most commonly observed types and forms of inclusions:

Class A (sulfides) : A single gray inclusion with high ductility and a wide range form ratio (length/width), generally with rounded ends.

Class B (alumina) : most undeformed, angular, small shape ratio (generally <3), black or bluish particles, arranged in a line along the rolling direction (at least 3 particles).

Class C (silicates) : A single black or dark gray inclusion with high ductility and a wide range form ratio (generally ≥3), with an acute Angle at the end.

Class D (spherical oxides) : non-deformed, angular or round, small shape ratio (generally <3), black or bluish, irregularly distributed particles.

Class DS (single particle spherical class) : round or nearly round, diameter ≥13μm of single particle inclusion.

Each type of inclusion is divided into two series of fine and coarse according to the different width of non-metallic inclusion particles.

4. Determination of inclusion content

The sample was magnified 100 times under the microscope, and the inclusion grade was evaluated in the field of view of a square with a side length of 71mm and an actual area of 0.5mm².

Method A is used for production inspection

The entire finish should be inspected. For each type of inclusion, the number of levels of the standard picture corresponding to the worst field of view on the examined surface is recorded by fine and coarse series.

For individual inclusions whose length exceeds the side length of the field of view (0.71mm), or whose width or diameter is greater than the maximum of the coarse system (see Table 2), they should be assessed as super-size inclusions and recorded separately. However, these inclusions should still be included in the rating of the field of view.

For strip inclusions that are not in a straight line, if the longitudinal distance between the two inclusions is less than or equal to 40μm and the transverse distance along the rolling direction is less than or equal to 10μm, it shall be regarded as an inclusion.

If the widths of inclusions within an inclusion are different, the maximum width of the inclusion shall be regarded as the width of the inclusion.

If the inclusion length is too long or the width is too large, it should be recorded separately and included in the field of view for rating (the length of the extra long is 0.71mm).

5. Inclusion grade calculation (measurement rating picture grade of inclusions above 3)

Class A sulfide (length L expressed by μm) : Lg(i)=[0.560 5lg(L)]-1.179

Class B alumina: Lg(i)=[0.462 6lg(L)]-0.871

Class C silicate: Lg(i)=[0.480 7lg(L)]-0.904

Class D spherical oxides, where n is the amount in each field of view: Lg(i)=[0.5lg(n)]-0.301

DS class single particle spherical oxide, d is diameter: i=[3.311g(d)]-3.22

CANGZHOU GALAXY STEEL PIPE CO., LTD

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Various structures and inclusions in steel (Chapter Two)

6 Bainite

The phase transition product formed between 350℃ and 550℃ of supercooled austenite is called the bainite structure, with the grain boundary as the axis of symmetry, and is like a feather. This feather structure is composed of parallel acicular or lamellar ferrite and short strip cementite distributed between the ferrite, the direction of the short strip cementite is roughly parallel to the lamellar ferrite. The strength of upper bainite is higher than that of pearlite formed from steel of the same composition. The upper bainite hardness is between 40-45 HRC.

7 Lower bainite

Below 350℃, the acicular structure formed between Ms points and above is called lower bainite. Lower bainite is a needle ferrite matrix with very fine carbide sheets distributed on it. In the crystal is needle-like, both ends pointed, needles basically do not cross, but can be exchanged, and tempered martensite is not easy to distinguish. High-carbon high-alloy needles are relatively thin, blue-black in color, and low-carbon low-alloy steel is gray.

Compared with upper bainite, lower bainite not only has higher strength, hardness and wear resistance, but also has good plasticity and toughness. The strength of lower bainite is similar to that of tempered martensite at corresponding temperature, and the hardness is between 45 and 55HRC.

8 granular bainite

The bulk ferrite contains some cementite particles and "islet" structures. It usually appears in low carbon and low alloy.

9 martensite

Martensite is a supersaturated solid solution of carbon in α-Fe.

When the carbon content of steel is low, when the steel is rapidly cooled from the austenitic state, it is transformed into a sheet martensite in the continuous cooling process, also known as low carbon martensite. Lath martensians are thin wooden strips arranged parallel to each other on a crystal face.

Lath martensite has higher strength and toughness, higher fracture toughness and lower cold brittle transition temperature.

High-carbon austenite forms sheet martensite, also known as acicular martensite. The martensite grows into the austenite crystal to form needles, and the needles are at a certain Angle, intersecting each other but not passing through. Martensite in steel has high strength and high hardness, but the plasticity and toughness are very low and the brittleness is large. The strength and hardness of martensite increase with the increase of carbon content in martensite, but the ductility and toughness decrease sharply.

10 Tempered martensite

When martensite is tempered at low temperature (150 ~ 250℃), most of the suoversaturated carbon is precipited from the inside of martensite in the form of highly dispersed cementite and carbide, so that martensite sheet is vulnerable to corrosion during the preparation of metallography sample, and appears black under the microscope, and carbide particles cannot be distinguished under the optical microscope. Such martensite is called tempered martensite. Tempered martensite retains the directionality of quenched martensite needles. In terms of performance, tempered martensite still has high hardness and high wear resistance, while reducing the internal stress and brittleness of quenching. Tempered hardness is generally 58-64HRC.

11 Temper troostite

The tissue obtained after tempering martensite at medium temperature (350℃ ~ 500℃) is called tempered troostite. Tempered troostite is a mixture of ferrite and cementite, in which cementite is fine granular distribution, martensitic needle direction is obvious, dark color, can not distinguish carbide particles at 500 times.

Tempered troostite has high elastic limit, yield limit and high toughness and plasticity. Medium temperature tempering is mainly used for various springs and hot dies, and the hardness after tempering is generally 35-50HRC.

12 tempered sorbite

The product of martensite tempering at high temperature (500℃ ~ 650℃) is called tempered sorbite. Tempered sorbite is also a mixture of ferrite and cementite, in which the cementite particles are coarser than those in tempered troosite and are clearer under metallographic microscopy. The purpose of getting tempered sorbite is to obtain good comprehensive mechanical properties of strength, plasticity and toughness. Hardness after tempering is generally 20-35 HRC.

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Quenched and Tempered Steel Performance Characteristics and Applications (Chapter Two)

 Composition characteristics

(1) Medium carbon: the carbon mass fraction is generally between 0.25% and 0.50%, and the majority is 0.4%;

(2) Add elements to improve hardenability Cr, Mn, Ni, Si, etc. : In addition to improving hardenability, these alloy elements can also form alloy ferrite and improve the strength of steel. For example, the performance of 40Cr steel after tempering treatment is much higher than that of 45 steel;

(3) Add elements to prevent the second type of tempering brittleness: alloy tempered steel containing Ni, Cr and Mn, which is easy to produce the second type of tempering brittleness when tempered at high temperature and cooled slowly. The addition of Mo and W in steel can prevent the second type of tempering brittleness, and the appropriate content is: the mass fraction of Mo is 0.15% to 0.30%, or the mass fraction of W is 0.8% to 1.2%.

Classification and Application:

1, medium carbon steel: on behalf of steel grades 30, 35, 40, 45, ML30, ML35, ML40, ML45, has a more stable room temperature performance, for small and medium-sized structural parts, fasteners, drive shaft, gear and so on.

45 steel hot and cold processing performance is good, good mechanical properties, and low price, wide source, so widely used. Its biggest weakness is low hardenability, large cross-section size and relatively high requirements of the workpiece should not be used.

2, manganese steel: representing steel 40Mn2, 50Mn2. It has overheating sensitivity, high temperature tempering brittleness, water quenching easy cracking, and higher hardenability than carbon steel.

3, silicon manganese steel: on behalf of steel 35SiMn, 42SiMn. High fatigue strength, decarburization and overheating sensitivity and temper brittleness. It is used to manufacture gears, shafts, rotating shafts, connecting rods and worm with medium speed and medium and high load but little impact, and can also manufacture fasteners below 400℃.

4, boron steel: representing steel 40B, 45B, 50BA, ML35B. High hardenability, comprehensive mechanical properties are higher than carbon steel, and 40Cr is used for manufacturing parts and fasteners with small cross-section size.

5, manganese boron steel: representing steel 40MnB. Hardenability slightly higher than 40Cr, high strength, toughness and low temperature impact toughness, temper brittleness. 40MnB is commonly used to replace 40Cr to manufacture large-section parts and 40CrNi to manufacture small parts. 45MnB replaces 40Cr and 45Cr; 45Mn2B replaces 45Cr and partially replaces 40CrNi and 45CrNi as an important shaft, and ML35 MnB is also used for fastener production.

6, manganese vanadium boron steel: representing steel 20 MnVB, 40MnVB, The tempering performance and hardenability are better than 40Cr, the tendency to overheat is small, and there is temper brittleness. It is commonly used to replace 40Cr, 45Cr, 38CrSi, 42CrMo and 40CrNi to manufacture important tempered parts, and also used for small and medium-sized bolts below 10.9 level, ML20 MnVB.

7, manganese tungsten boron steel: representing steel 40MnWB. Good low temperature impact performance, no tempering brittleness. It is equivalent to 35CrMo and 40CrNi and is used to manufacture parts below 70mm.

8, silicon manganese molybdenum tungsten steel: representative of steel 35SiMn2MoW. High hardenability, calculated by 50% martensite, water quenching diameter 180, oil quenching diameter 100; The tendency of quenching cracking and tempering brittleness is small. It has high strength and toughness. Can replace 35CrNiMoA, 40CrNiMo, used to manufacture large section, heavy load shaft, connecting rod and bolt.

9, silicon manganese molybdenum tungsten vanadium steel: representative of steel 37SiMn2MoWVA. Water quenching diameter 100, oil quenching diameter 70; Good tempering stability, low temperature impact toughness, high high temperature strength, tempering brittleness is also small, used for manufacturing large section of shaft parts.

10, chromium steel: 40Cr and ML40Cr as representatives. Good hardenability, water quenching 28-60mm, oil quenching 15-40mm. High comprehensive mechanical properties, good low temperature impact toughness, low notch sensitivity, temper brittleness. Used in the manufacture of shafts, connecting rods, gears and bolts.

11, chromium silicon steel: on behalf of steel 38CrSi. Hardenability is better than 40Cr, the strength and low temperature impact are higher, the tempering stability is better, and the tempering brittleness tendency is larger. It is often used in the manufacture of 30-40mm shafts, bolts and gears with small modulus.

12, chromium molybdenum steel: representing steel 30CrMoA, 42CrMo, ML30CrMo, ML42CrMo. Water quenching 30-55mm, oil quenching 15-40mm; High room temperature mechanical properties and high high temperature strength, good low temperature impact; No temper brittleness. Used for manufacturing parts with large sections, high load bolts, gears and flanges and bolts below 500℃; Ducts and fasteners below 400℃. 42CrMo has higher hardenability than 30CrMoA and is used to make parts with higher strength and larger sections.

13, chromium manganese molybdenum steel: representing steel 40CrMnMo. Oil quenching diameter 80mm, with high comprehensive mechanical properties, good tempering stability. Used for manufacturing heavy load gear and shaft parts with large cross sections.

14, manganese molybdenum vanadium steel: representing steel 30Mn2MoWA. It has good hardenability: water quenching reaches 150mm, and the heart is composed of upper and lower bainite plus a small amount of martensite; Oil quenching 70mm, more than 95% of martensite in the heart; Good low temperature impact toughness, low notch sensitivity and high fatigue strength. For the manufacture of important parts under 80mm.

15, chromium manganese silicon steel: on behalf of steel 30CrMnSiA. Water quenching 40-60mm (95% martensite), oil quenching 25-40mm. High strength, impact toughness, temper brittleness. Used to manufacture high pressure blower blades, valve plates, clutch friction plates, shafts and gears.

16, chrome-nickel steel: representing steel 40CrNi and 45CrNi. Water quenching up to 40mm, oil quenching 15-25mm; Good comprehensive mechanical properties, good low temperature impact toughness, low tempering brittleness tendency. 30CrNi3A has high hardenability, good comprehensive mechanical properties, white spot sensitivity and temper brittleness. It is used to manufacture crankshaft, connecting rod, gear, shaft and bolt with large cross section.

17, chromium nickel molybdenum steel: representing steel 40CrNiMoA. It has excellent comprehensive mechanical properties, high low temperature impact toughness, low notch sensitivity and no temper brittleness. It is used to manufacture large crankshafts, shafts, connecting rods, gears, bolts and other parts with large forces and complex shapes.

18, chromium nickel molybdenum vanadium steel: on behalf of steel 45CrNiMoVA. High strength, good tempering stability, oil quenching up to 60mm (95% martensitic). It is used to manufacture elastic shafts and torsion shafts of heavy-duty vehicles under vibration loads.

Email: sale@galaxy-steel.com

 

Wechat: LXF13931739696.  Whatsapp: 008613931739696

 

Website:  https://www.friend-steelpipes.com/  https://www.cz-steelpipe.com/

 

CANGZHOU GALAXY STEEL PIPE CO., LTD

 

China Reliable Steel Pipe and Pipe Fittings Manufacturer and Supplier

 

My company hot sale steel pipe and pipe fittings:

 

ASTM A106 B seamless pipes,（1” to 24” sch40, sch80),

API 5L GR.B steel pipes, (seamless pipe, LSAW pipe, SSAW pipes, 1” to 110” sch 40, sch80),

Boiler tubes,( 20G. 12Cr1MoVG, ASTM A335 P5, P9, P11),

ASTM A333 GR.6 seamless pipes, ( 1” to 24” sch40, sch80, sch160),

TP304/304L, TP316/TP316L, stainless steel pipes, (1” to 48” sch10, sch20, sch40, sch80),

API 5L X65, X70 PSL2 LSAW welded pipes, (20” to 60” sch40, sch80, sch160),

API 5CT seamless casing, and tubing, (2 3/8” to 20” K55, J55, N80, L80, P110),

Pipe fittings, elbow, reducer, ,tee,cap, flange