"Fastener Materials And Heat Treatment" Technology Hot Spot

1. what is the added elements of steel, why is it easy to quench and crack?

GB/T3098.1 ~ 2010 standard for 8.8 ~ 10.9 grade bolts used in the material there are clear provisions for the need for heat treatment of high-strength bolts steel sulfur and phosphorus maximum content limit, from 0.035% down to 0.025%, which has "added elements of carbon steel (such as boron or manganese or chromium)". A class, and instead of the original standard "low carbon alloy steel". It is reported that such steel using "scrap" as raw materials, electric furnace smelting, often too much residual alloying elements, control is very difficult, especially some of the impurity elements difficult to remove in the smelting, resulting in the final product quality can not be guaranteed.

Generally refers to 45 #, 40Cr steel, this type of steel in China's long service life. Especially S, P content are in 0.025% to 0.035%, there are other impurities Cu copper, Pb lead, Sn tin, Se selenium, when the harmful elements in the steel P, S increase, low times the organization of the detection of the center of the sparsity, general sparsity, square segregation of the level of the level of the 2 ~ 3, white spots, shrinkage, bubbles, flap and other defects can be seen to exist. Due to the lower price, many fastener companies also like to use. Quenching cracks occur from time to time in the bolt tempering accidents, mainly due to impurity elements and non-metallic inclusions are more caused by any measures taken during heat treatment are unavoidable cracking. For important products, it is recommended that generally do not use or avoid the use of 45 #, 40Cr steel.

2.the current market, 20MnTiB, ML20MnTiB boron steel can be manufactured 10.9 high-strength bolts, but what is the reason for the decline in mechanical properties after the re-tempering test?

Bolts in the fasteners in the largest amount of use, manufacturing materials are mainly steel, stainless steel, titanium alloys and non-ferrous metals. At present, in addition to a small number of large specifications using warm upsetting (warm extrusion), cutting manufacturing, most of the choice of cold upsetting (cold extrusion) molding process manufacturing. The final mechanical properties of the bolt is determined by the quality of the material used to manufacture the fastener, and the lower limit of carbon content of 20MnTiB and ML20MnTiB is lower than 0.20%, which can not satisfy the standard requirements of the material; 20MnTiB steel bolts strength ≥ 1040MPa, hardness of 32-39HRC requirements, is achieved by reducing the tempering temperature, when using 425 ℃ re-tempering temperature After the test hardness and strength decline, can not meet the performance requirements of the minimum tempering temperature in the standard. From production practice can be seen, the excellent overall mechanical properties of the bolt, not only depends on the heat treatment process and metallurgical organization, more importantly, should have a good chemical composition match.

20MnTiB and ML20MnTiB steel should first ensure that the carbon content of 0.22% to 0.24% or more, in order to maximize the hardenability, hardness after quenching ≥ 45 ~ 47HRC, in order to achieve an effective depth of hardened layer, in order to meet the minimum tempering temperature of 425 ℃ in the standard requirements.

3.U.S. hexagon socket pan head screws 1/4-20 × 1/2, material: SCM435, mechanical properties 12.9 grade, the required torque of 20N-m, the heat treatment torque is only 17 ~ 18N-m, what is the cause of?

This problem is very critical. SCM435 steel carbon content to take the lower limit, this problem can not be solved, SCM435 steel its carbon content to the upper limit of the upper limit or the use of 42CrMoA, SCM440 steel, this problem is well solved. For this reason, all steel should be based on the bolt grade to choose the appropriate carbon content, the amount of carbon in the steel changes, the quenching temperature will change, not only the tensile strength, hardness will change, and torque will also change. In addition, re-tempering heat treatment can also meet the technical requirements.

4.with martensitic stainless steel 30Cr13 old grade 3Cr13 made of closed circular spring sheet, thickness 0.75mm, hardness requirements of 39 ~ 43HRC, elasticity test requires room temperature rebound more than 10 times. The current process can meet the hardness requirements, but the elasticity pass rate is only about 30%. On the premise of not changing the material, how to ensure the elasticity of the product?

This involves a problem of composition, we engage in heat treatment if we do not put the composition in the first place that is not good. Can use carbon content ≥ 0.32% to 0.35% of the steel, banded tissue segregation should be controlled below 2 levels, which control the level of non-metallic inclusions, such as sulfide class, alumina class, globular oxides class are not greater than 1.5 levels (including coarse and fine system). If the elasticity is not enough, consider adding a 300 ~ 310 ℃ re-tempering process, and then fast cooling. Can try.

5. Which austenitic stainless steel grade is used for stainless steel weld nuts used in automobiles?

Austenitic stainless steel has good general corrosion resistance, excellent resistance to all-round corrosion in many media, but more sensitive to intergranular corrosion and stress corrosion cracking. Any heat treatment process will not harden it; however, the mechanical properties of the steel can be improved due to the increase in N content or cold forming strengthening. Austenitic steels have good weldability and also high low-temperature impact toughness, as well as high safety against embrittlement.

12Cr18Ni9 (SUS302) old grade 1Cr18Ni9. general-purpose standard 18-8 type CrNi stainless steel, corrosion resistance is better than the same type of 12Cr17Ni7 (SUS301) steel, cold-worked to make the tensile strength of the tensile strength has been greatly improved, but elongation is not as good as the 12Cr17Ni7, used for general-purpose corrosion-resistant bolts that need to be machined welded nuts, Self-tapping screws.

Y12Cr18Ni9 (SUS303) old grade Y1Cr18Ni9. 18-8 austenitic stainless steel on the basis of increasing the amount of S to improve the cutting performance, Mo and Zr elements can be added, suitable for batch cutting fasteners, especially when the bar processing nuts.

022Cr19Ni10 (SUS304L) old grade 00Cr19Ni10. In addition to slightly lower strength, other properties and 06Cr19Ni10 (SUS304) the same, mainly used for the need to weld and weld and do not carry out the solution treatment of the nut.

6.the future direction of automobile development is lightweight, energy saving, safety and environmental protection. In the assembly of indispensable threaded connections, the pursuit of higher design stress, lightweight requirements, more reasonable fastening technology. In order to reduce the risk of delayed fracture of high-strength fasteners, what kind of stainless steel is used to manufacture engine bolts?

General use of precipitation hardening stainless steel, also known as age-hardening steel, the matrix is austenitic or martensitic organization, and can be hardened by precipitation hardening treatment to make it hard (strong) stainless steel. This type of steel in the high corrosion resistance at the same time also has a high strength; its corrosion resistance is not only related to the composition, and heat treatment is closely related to the precipitation of fine phases, aging reaction on the corrosion resistance are harmful. The high strength of the steel is due to the final heat treatment at lower temperatures from the martensite phase precipitation of intermetallic compounds, and because of the high strength characteristics of this type of steel, the use of hydrogen embrittlement and stress corrosion may also occur.

05Cr17Ni4Cu4Nb (SUS630) old grade 0Cr17Ni4Cu4Nb, commonly used precipitation hardening stainless steel, it has both the high strength of martensitic stainless steel and austenitic stainless steel corrosion resistance, good overall performance, at 315 ℃ still have high strength and corrosion resistance, used for both the requirements of high strength, but also requires good corrosion resistance of the car bolt. 07Cr17Ni7Al (SUS631) old grade 0Cr17Ni7Al, plasticity, easy to harden, at higher temperatures still have high strength and corrosion resistance, especially cold, hot deformation performance is excellent, heat treatment to strengthen the effect is significant, suitable for variant molding of high-strength, corrosion-resistant automotive bolts. 07Cr15Ni7Mo2Al (SUS632) old grade 0Cr15N-i7Mo2Al, in 07Cr17Ni7Al (SUS631) based on the addition of alloying elements of molybdenum to ensure that the heat treatment to get higher strength.

06Cr15Ni25Ti2MoAlVB (SUS660) old grade 0Cr15Ni25Ti2-MoAlVB. iron-nickel based alloy, which at high temperatures at the same time has high strength and good corrosion resistance, good cutting performance and hot working properties, can be used at a high temperature of 650 ~ 700 ° C for heat and corrosion resistance of the stressed automobile engine bolts.

In addition, martensitic stainless steel is a steel with good thermal stability and thermal strength, such as 42Cr9Si2 and 40Cr10Si2Mo steel. High strength at high temperatures, also known as martensitic heat-resistant steel.GB/T 1221-2007 "heat-resistant steel bar" standard refers to the 42Cr9Si2 old grade 4Cr9Si2 and 40Cr10Si2Mo old grade 4Cr10Si2Mo steel is an important bolt material on the engine, the work of the engine to withstand high mechanical loads and thermal loads.