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

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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.

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CANGZHOU GALAXY STEEL PIPE CO., LTD

 

China Reliable Steel Pipe and Pipe Fittings Manufacturer and Supplier

 

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

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

  Quenched and Tempered Steel generally refers to medium carbon steel with a carbon content of 0.3-0.6%. The metallography structure after tempering is tempered sorbite. Tempered steel is used extensively for structural parts on various machines. The most widely used tempered steels are chromium tempered steels (such as 40Cr, 40CrSi), chromium manganese tempered steels (such as 40CrMn), chromium nickel tempered steels (such as 40CrNiMo, 37CrNi3A), and boron-containing tempered steels.

The heat treatment process is quenched into martensite after heating at a certain temperature above the critical point, and tempered at 500℃-650℃. The microstructure after heat treatment is tempered sorbite. The structure has a good combination of strength, plasticity and toughness.

The quality requirements of tempered steel, in addition to the general metallurgical aspects of low and high microstructure requirements, mainly for the mechanical properties of steel and cold brittleness transition temperature, fracture toughness and fatigue resistance closely related to working reliability and life. Under specific conditions, it is also required to have wear resistance, corrosion resistance and certain heat resistance. Because the tempered steel finally adopts high temperature tempering, the stress in the steel can be completely eliminated, and the hydrogen brittleness of the steel is small, the notch sensitivity is low, and the brittleness resistance is large, but there is also a unique high temperature tempering brittleness.

Most tempered steels are medium carbon alloy structures with yield strengths (σ0.2) in the 490-1200Mpa range. The tempered steel with welding performance as the outstanding requirement is low-carbon alloy structural steel, the yield strength (σ0.2) is generally 490-800Mpa, and has high plasticity and toughness. A small number of precipitation-hardened tempered steels with yield strength (σ0.2) can reach more than 1400Mpa, belonging to high strength and ultra-high strength tempered steels.

Commonly used Quenched and Tempered Steel is divided into 4 categories according to hardenability and strength:

① low hardenability steel mixing; ② medium hardenability tempered steel; ③ Tempered steel with high hardenability; ④ Tempered steel with high hardenability.

1. Effect of alloying elements on mechanical properties

By adjusting the content of alloying elements that increase the hardenability of steel, the same hardenability can be obtained and the same tensile strength and yield strength can be obtained. Therefore, when selecting alloying elements, priority should be given to elements that increase the hardenability and have a significant effect on the price, such as boron, manganese, chromium, etc. However, the tempering temperature used for steels with different alloying elements to be tempered to the same hardness is different, that is, the tempering resistance of various steels is different.

When the steel with the same hardenability is tempered to the same hardness, the tensile strength and yield strength are basically the same, but the brittleness failure tendency is very different, and the low temperature impact test is particularly obvious. The relationship between hardness and fatigue limit of tempered steel with different composition is different. When the hardness is below 35HRC, the fatigue limit and hardness are in a linear relationship, and the fluctuation range of fatigue limit is 130MPa. When the hardness exceeds 35HRC, the fluctuation range of fatigue limit becomes wider. For example, when the hardness is 55HRC, the fluctuation range of the fatigue limit is 380MPa.

2. Determination of hardness of tempered parts

The hardness of tempered parts can reflect the yield strength and tensile strength of the parts when the quenching conditions are the same.

The determination of hardness of tempered parts must take into account the requirements of the manufacturing process and the load conditions when used. From the manufacturing process, it is hoped that the parts are conditioned in the blank state, and then the cutting and assembly are carried out. In this way, the deformation and decarbonization caused by the heat treatment of the parts are eliminated in the later cutting process. However, the hardness of the parts using this manufacturing process cannot be too high, generally not more than 300HB, and the individual does not exceed 350HB, otherwise it is unfavorable to the cutting process. Parts that require higher hardness (such as some auto shafts requiring hardness of 341 ~ 415HB) can only be cut first, and then tempered, when the parts are heated, decarbonization and deformation should be prevented, and sometimes the straightening process should be increased after heat treatment. For parts produced in small batches or single pieces, the hardness allowed by cutting can be appropriately increased.

Determining the hardness of tempered parts must also take into account the characteristics of production, small batches of single production products, different parts can be selected for different hardness, mass production factories hope that the hardness range of most parts is consistent or fixed in several hardness ranges, which is very convenient for the organization of heat treatment production.

From the point of view of the use of parts, it is necessary to pay attention to the working conditions of parts and the shape of parts when determining the hardness of tempered parts. Generally speaking, the hardness value is high, the tensile strength, the yield strength and the fatigue strength of the smooth sample are high, but the plastic index is reduced, the brittleness failure tendency and the sensitivity of stress concentration are increased, therefore, when there is a notch on the part that plays the role of stress concentration (spline, groove or cross section change), in order to make the stress distribution uniform and reduce the stress concentration phenomenon. At this time, lower hardness can obtain higher fatigue properties.

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CANGZHOU GALAXY STEEL PIPE CO., LTD

 

China Reliable Steel Pipe and Pipe Fittings Manufacturer and Supplier 

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