Chapter 6 Heat Treatment

1、 Heat treatment method: Different heat treatment methods can be selected according to the object and purpose.

Quenched and tempered steel: High temperature tempering after quenching (500-650 ℃)

Spring steel: quenched and tempered at medium temperature (420-520 ℃)

Carbonized steel: carburizing, quenching, and low-temperature tempering (150-250 ℃)

After quenching low carbon and medium carbon (alloy) steel into martensite, the general rule is that the strength decreases while the plasticity and toughness increase with the increase of tempering temperature. However, due to the different carbon contents in low and medium carbon steels, the degree of influence of tempering temperature on them varies. So in order to obtain good comprehensive mechanical performance, the following approaches can be taken:

(1) Select low-carbon (alloy) steel, quench it, and then temper it at a low temperature below 250 ℃ to obtain low-carbon martensite. In order to improve the surface wear resistance of this type of steel, only by increasing the carbon content of each surface layer, that is, surface carburizing, is generally referred to as carburized structural steel.

(2) Adopting medium carbon steel with high carbon content, quenching and tempering at high temperature (500-650 ℃) (so-called quenching and tempering treatment) to maintain sufficient strength under high plasticity conditions, this type of steel is generally referred to as quenched and tempered steel. If one wishes to achieve high strength but prefers to reduce plasticity and toughness, low-temperature tempering can be adopted for gold containing quenched and tempered steel with low carbon content, resulting in the so-called "ultra-high strength steel".

(3) Steel grades with carbon content between medium carbon and high carbon (such as 60,70 steel) and some high carbon steels (such as 80,90 steel), if used to manufacture springs, are quenched and tempered at medium temperature to ensure high elastic limit, yield limit, and fatigue limit.

2、 Homework process:

（1） Tempered and tempered steel:

1. Preheating treatment:

(1) The purpose of normalizing is to refine the grain size, reduce the degree of banding in the structure, and adjust the hardness for easy mechanical processing. After normalizing, the steel has equiaxed fine grains.

2. Quenching: Heat the steel body to around 850 ℃ for quenching. The quenching medium can be selected according to the size of the steel part and the hardenability of the steel. Generally, water, oil, or even air quenching can be chosen. Steel in a quenched state has low plasticity and high internal stress.

3. Tempering:

(1) In order to give steel high plasticity, toughness, and appropriate strength, steel is subjected to high-temperature tempering at around 400-500 ℃. For steel that is more sensitive to tempering brittleness, it must be rapidly cooled after tempering to suppress the occurrence of tempering brittleness.

(2) If the parts are required to have particularly high strength, they can be tempered at around 200 ℃ to obtain a medium carbon tempered martensitic structure.

（2） Spring steel:

1. Quenching: Oil quenching at 830-870 ℃.

2. Tempering: Temper at around 420-520 ℃ to obtain tempered martensite structure.

（3） Permeable carbon steel:

1. Carburizing: A type of chemical heat treatment that involves the infiltration of C element into the surface of a steel component in an active medium containing a certain chemical element at a certain temperature. Preheating (850 ℃) carburizing (890 ℃) diffusion (840 ℃) process

2. Quenching: Carbon and low-alloy carburizing steel are generally quenched directly or in one go.

3. Tempering: Low temperature tempering is used to eliminate internal stress and improve the strength and toughness of the carburized layer. In our production, the tempering temperature for tapping screws is around 360 ℃, and for self drilling screws (wall studs) it is around 200 ℃. After that, they are cooled to 34-35 ℃ and 39-40 ℃ respectively.

Part VI Standard Specifications

Standards are norms, and every country and department has its own standards. At present, the most commonly used standards in our daily business are as follows:

GB - Chinese National Standard (GB) ANSI - American National Standard (ANSI)

DIN - German National Standard (German Standard) ASME - American Society of Mechanical Engineers Standard

JIS - Japanese National Standard (Japanese Standard) BSW - British National Standard

GB - National Standard is one of the many standards in China, in addition to industry standards, professional standards, and departmental standards. National standards are divided into GB (mandatory standards), GB/T (recommended standards), and GBn (internal national standards). What we usually see, such as GB30, GB5783, etc., are mandatory standards.

In addition to differences in basic dimensions such as head to edge and head thickness, the most important difference among the above standards is the difference in the threaded part. The threads of GB, DIN, JIS, etc. are all measured in millimeters (MM) and are collectively referred to as metric threads. Threads such as ANSI, ASME, etc., which are measured in inches, are called American standard threads. In addition to metric threads and American threads, there is also a BSW - British Standard, whose threads are also measured in inches, commonly known as Wyeth threads.

Metric threads are measured in millimeters (MM) and have a tooth tip angle of 60 degrees. Both American and British threads are measured in inches. The tooth tip angle of American thread is also 60 degrees, while the tooth tip angle of British thread is 55 degrees. Due to different units of measurement, the representation methods of various threads are also not the same. For example, M16-2X60 represents metric threads. His specific meaning is that the nominal diameter of the screw is 16MM, the pitch is 2MM, and the length is 60MM. For example, 1/4-20X3/4 represents the British thread, and his specific meaning is that the nominal diameter of the screw is 1/4 inch (one inch=25.4MM), with 20 teeth on one inch and a length of 3/4 inch. In addition, when referring to American screws, UNC and UNF are usually added after representing British screws to distinguish between American coarse teeth and American fine teeth.

In our daily domestic sales business, the standards we most commonly encounter are GB (National Standard) and DIN (German Standard) products. We mainly come into contact with the following standards: GB30; GB5783; GB5782; GB62; GB6170; GB818; GB819; GB845; GB846; GB70; DIN912; DIN933; DIN931, etc. At present, GB30 (old national standard) has been replaced by GB5783 (new national standard) in the standard book. GB52 (old national standard) has been replaced by GB6170 (new national standard) in the standard book.

In 1986, our country formulated a new standard for standard components, commonly known as the new standard in business. The most commonly used ones are GB5780, GB5781, GB5782, GB5783, and GB5784. GB5780 is a hexagonal head thick rod half tooth screw with a precision level of C, which can be replaced by GB5782 (GB5782 is a hexagonal head thick rod full tooth screw with a precision level of A and B). GB5781 is a hexagonal head full tooth screw with a precision level of C. GB5783 can be used as a substitute (GB5783 is a hexagonal head full tooth screw with accuracy levels of A and B). GB5784 is a hexagonal screw with a thin rod and half tooth.

The difference between the new label and the old label is that the M8, M10, M12, M14, and M22 series products have differences in the width of the edges. Except for the new M22 series products, the head edges of the new label products M8, M10, M12, and M14 are 1MM smaller than those of the old label. They are 13mm, 16mm, 18mm, and 21mm respectively. However, for the new M22 series products, the new label has a 2mm larger edge than the old label, so special attention should be paid. For the thickness of the head, there is a slight difference between the new and old standards, which can be used interchangeably in situations where the requirements are not very strict.

The difference between the new standard and the German standard lies in the product specifications of M10, M12, M14, and M22, which differ in the width of the edges. The head to edge new standard of M10, M12, M14 should be 1MM smaller than the German standard. The new product of M22 has a head edge width that is 2MM wider than the marked edge width, and all other products can be used interchangeably.

For hexagonal nuts, commonly used standards include GB52, GB6170, GB6172, and DIN934. The main difference between them is that GB6170 is thicker than GB52, GB6172, and DIN934, commonly known as thick nuts. In addition, there is a difference in the edges. In the M8 nut series, the edges of DIN934, GB6170, and GB6172 are 13mm, which is 1MM smaller than the 14mm edge of GB52. For M10 nuts, the edges of DIN934 and GB52 are 17mm, which is 1MM larger than the edges of GB6170 and GB6172. For M12 nuts, the edges of DIN934 and GB52 are 19mm, which is 1MM larger than the 18mm edge of GB6170 and GB6172. For M14 nuts, the opposite edge of DIN934 and GB52 is 22mm, which is 1MM larger than the opposite edge of GB6170 and GB6172, which is 21mm. In addition, the M22 nut has a 32MM opposite edge to DIN934 and GB52, which is 2MM smaller than the 34MM opposite edge to GB6170 and GB6172. GB6170 and GB6172 have exactly the same edge width except for their different thicknesses. All other specifications can be used interchangeably without considering thickness.

In terms of internal hexagon, there are two versions in the national standard, one is GB70-76, version 76, and the other is GB70-85, version 85. If the DIN912 standard is implemented, attention should be paid to the differences in actual business operations. GB70-85 completely overlaps with DIN912, so there is no difference when using the new standard. The main difference is between GB70-76 and DIN912: for M8 series internal hexagon products, the round head diameter of GB70-76 is 12.5MM, which is slightly smaller than DIN912's 13.27MM, and for M10 series internal hexagon products, the round head diameter of GB70-76 is 15MM, which is slightly smaller than DIN912's 16.27. The inner hexagon of the M12 series, GB70-76 has a round head diameter of 18mm, which is slightly smaller than the opposite side of DIN912 at 18.27. In addition, the round head diameter of the inner hexagon GB70-76 of the M16 and M20 series is 0.33mm smaller than DIN912, 24mm and 30mm respectively. DIN912 is 24.33mm and 30.33mm respectively. In addition, the width of the inner edge between the hexagon of the old standard and the German standard varies due to different standards. The inner edge of GB70-76 is smaller and should also be taken into account in business operations.

In addition, there are some differences in the carriage screws that may be used in daily life. Here is also an explanation. In the national standard, there are two standards for carriage screws, namely GB12 (small half round head square neck screw) and GB14 (large half round head square neck screw). The German standard DIN603 is also commonly used in the market. Now distinguish these three: for round neck, when comparing the same specification, it is: GB12<GB14<DIN603. Usually, when using carriage screws, a large and thick head and neck are required, so the standard of DIN603 carriage screws fully meets the requirements.