Article you must not miss - Metallographic inspection method of high-speed steel after heat treatment

High-speed steel, also known as high-speed tool steel or sharp steel, is a tool steel with high hardness, high wear resistance and high heat resistance. Its composition is complex and contains carbide-forming elements such as tungsten, molybdenum, chromium, vanadium, and cobalt. . The heat treatment process of high-speed steel is relatively complex and must go through a series of processes such as annealing, quenching, and tempering. Due to its good process performance and good combination of strength and toughness, high-speed steel is mainly used to manufacture complex thin blades and impact-resistant metal cutting tools. It can also be used to manufacture high-temperature bearings and cold extrusion dies. High-speed steel is widely used, so it is essential to conduct quality inspection of high-speed steel. High-speed steel generally does not undergo tensile strength testing, but mainly metallographic and hardness testing. This article briefly introduces the metallographic examination of high-speed steel after heat treatment.

1. Inspection of the quenching overheating degree of high-speed steel

The degree of quenching superheating of high-speed steel is measured by the change in carbide morphology. The degree of overheating can be divided into five levels according to the degree of elongation, tailing and reticularization of carbides along the grain boundaries.

When high-speed steel cutting tools are severely quenched and overheated, their brittleness increases and they are prone to chipping or even failure to cut. The allowable overheating degree of various cutting tools should comply with national regulations.

It is better to inspect the degree of overheating of high-speed steel after tempering. After tempering, the matrix of the steel is corroded and turns black, which can more accurately determine the level of overheating.

2. Inspection of tempering degree of high-speed steel

After normal tempering of high-speed steel, the matrix structure is dark brown. When tempering is insufficient, the color of the matrix becomes lighter, and sometimes grain boundaries may even appear. Based on the ability of the matrix structure to accept erosion, it can be judged whether the tempering is sufficient.

The inspection of tempering degree is greatly affected by the concentration of corrosive agent, corrosion temperature and time. The corrosion specifications for 4% nitric alcohol (mass fraction) solution are:

2O-25 ℃，≤3min 

26-30 ℃, ≤2min 

  >30 ℃, ≤lmin 

Levels 1 and 2 are fully tempered and normal tempered, and level 3 is not fully tempered. The tempering levels of W series high speed steel and large size high speed steel are otherwise specified. Large-size high-speed steel has serious segregation of carbides. Where carbides gather, the austenite has a high degree of alloying, making it less susceptible to corrosion and prone to insufficient tempering.

3.Quenching grain size

The size of the quenched grain size of high-speed steel is a direct reflection of the quenching heating temperature and holding time. It is closely related to the strength, toughness, wear resistance, red hardness of the steel and the service life of the tool. It is particularly important for turning tools and large-size taper shanks. Tools such as cobalt heads that require wear resistance, high red hardness and small impact can have slightly larger quenching grain sizes. For small-size drill bits, center drills, taps, slender broaches and other tools that require higher strength and toughness, the quenching grain size can be slightly smaller.

There are regulations on the quenching grain size of various high-speed steel cutting tools. You can choose the best grain size according to the production and use of the cutting tools.

When checking the quenched grain size, carbide dissolution still needs to be considered. There is currently no good inspection method for carbide dissolution. It is usually measured by the number of carbide particles remaining in each grain after quenching.

During heat treatment of high-speed steel, some structural defects will inevitably occur, such as quenching decarburization structure, quenching over-burning structure, quenching naphthalene-like fracture structure, severe overheating, etc. Among them, the important characteristics of the naphthalene-like fracture structure are uneven grain sizes and individual coarse grains.

It is very necessary to understand and master the metallographic inspection method after heat treatment of high-speed steel. Mastering metallographic inspection methods and related knowledge can better produce high-quality and high-guaranteed workpieces and avoid the occurrence of some defects. Hope this article will be of great help to you.