Misunderstandings of Heat Treatment

This article introduces some examples of heat treatment misconceptions that you might encounter in real-world applications.

1. The product's heat treatment hardness can only reach 60 HRC, and 59 or 61 HRC are unacceptable?

We often encounter heat treatment orders where the hardness of the product must be within a specific value, with no deviation! For example, if the heat treatment hardness is required to reach 60 HRC, and your product only reaches 59 or 61 HRC after heat treatment, it will be considered unqualified. Little do people know that the allowable deviation for a Rockwell hardness tester is still 1 HRC.

2. Can't temper the quenched workpiece if it hasn't cooled to room temperature?

Some people believe that tempering can't be performed after quenching until it has cooled to room temperature. In fact, many steel grades, especially low- and medium-carbon steels, have martensitic transformation end points above room temperature. Cooling to room temperature actually makes them more susceptible to cracking. Therefore, tempering can be performed as soon as possible after quenching.

3. Must the quenched workpiece be tempered while still warm?

This practice is not advisable. The furnace temperature after quenching and before tempering should be determined based on the steel's martensitic transformation point! To prevent quenching cracking, avoid making arbitrary assumptions and using warm tempering as a blanket method!

4. The product has already been fully dimensioned, but is heat treatment required to ensure no deformation?

Some people, to save processing costs, complete all dimensioning before heat treatment, then perform heat treatment, quenching, and tempering. They require the heat treater to ensure no deformation during heat treatment, or to limit deformation to within the tolerance band of the final cold work step. The heat treatment process is essentially a stage of structural deformation, an accumulation of microscopic deformation.

5. Is it sufficient for heat treaters to master the iron-carbon equilibrium diagram?

Many documents indicate that the iron-carbon equilibrium diagram is crucial knowledge in heat treatment and forms the basis for developing heating processes for steel materials. Furthermore, they point out that heat treaters, in particular, must be proficient in the iron-carbon equilibrium diagram. The iron-carbon phase diagram represents the microstructure of an iron-carbon alloy at equilibrium, not the transformation diagram that leads to non-equilibrium structures such as martensite and bainite. The critical temperature parameters of the iron-carbon phase diagram are limited to carbon steel and cast iron, as well as non-alloy steel and alloy cast iron. The equilibrium diagrams of alloy steel and alloy cast iron differ significantly from the iron-carbon equilibrium diagram due to the addition of other alloying elements.

The iron-carbon equilibrium phase diagram is the result of extremely slow heating and cooling rates and is limited to iron-carbon alloy steels. This theoretical state is unlikely to be widely used in actual production. Microstructure transformations during actual heat treatments such as quenching occur at specific heating and cooling rates and do not fully reach equilibrium. Therefore, the iron-carbon equilibrium phase diagram is merely essential basic knowledge and a starting point for studying and learning heat treatment, not a phase diagram that can be directly applied in heat treatment processes.

For heat treaters, mastering the iron-carbon equilibrium phase diagram is only the beginning of heat treatment learning; it does not allow them to apply the iron-carbon equilibrium phase diagram to solve practical process problems. A good understanding of the iron-carbon phase diagram is only one of the basics of heat treatment.

6. Can annealing a workpiece form equiaxed grains?

Many people believe that equiaxed grains can be achieved during the annealing process of low-carbon steel. In fact, equiaxed grains are easily achieved in rimmed steel. However, achieving equiaxed grains in Al-killed steel is difficult. Especially when annealing parts that have undergone cold extrusion, the grains clearly exhibit a deformed extrusion structure! Even annealing temperatures above 950°C are difficult to achieve equiaxed grains.

7. Does lower hardness mean better and easier extrusion deformation?

People tend to assume that lower hardness means easier extrusion deformation. In the steel extrusion process, the pearlite spheroidized structure has the highest deformability, but this structure is generally harder than lamellar pearlite. Therefore, the technical requirement is that the original structure of the extruded part should be pearlite spheroidized, and lamellar pearlite, which has the lowest hardness, cannot be used.

8. Are surface wrinkles on aluminum alloy parts after heat treatment a sign of overheating?

After solution-aging treatment, there are two methods for determining whether aluminum alloy parts have been overheated during solution treatment: metallography and surface color. Judging overheating during solution treatment based on the workpiece's surface color and condition facilitates on-site resolution, but requires extensive experience. While metallography is accurate, it requires dissection of the actual part, which is destructive and can be wasteful.

Based on the workpiece's surface color and condition, the following conditions can be identified:

① The surface is dark gray; ② Small blisters appear on the workpiece surface; ③ Cracks appear with a rough fracture surface.

If any of these conditions are present, overheating is possible. This observation should only be made on workpieces after heat treatment. If abnormal surface conditions such as roughness, deformation, and wrinkles are observed on solution-aged parts after subsequent processing, it cannot be simply assumed that the aluminum alloy was overheated during heat treatment. Because aluminum alloys are still weaker than ferrous metals, the impact of subsequent processing steps must be analyzed. In particular, the effects of subsequent polishing and sandblasting on the surface cannot be ignored. When "water ripple"-like wrinkles appear locally on a workpiece, it's not due to overheating during heat treatment. Rather, it's due to excessive sandblasting pressure or prolonged sandblasting, resulting in a deformed layer on the aluminum alloy surface. These "water ripple" wrinkles are not a sign of overheating, but rather plastic deformation caused by impact. In this case, they should be considered sandblasting defects!

9. The manual states that a certain hardness can be achieved through heat treatment and quenching. Why can't you achieve this hardness?

Some people argue that the hardness range specified in the manual was chosen during design, so why can't you achieve this hardness through heat treatment?

For example, if a large part is made of 60Si2Mn spring steel, due to the large thickness of the actual workpiece, heat treatment is no longer effective to achieve the required hardness. The manual states a hardness of 58-60 HRC. However, this is unattainable based on the actual workpiece. The only option is to lower the heat treatment requirements.

The heat treatment hardness is determined by several factors: material grade, mold size, workpiece weight, shape and structure, and subsequent processing methods. After heat treatment, molds do not always have the same hardness inside and out. The material and design dimensions should be selected based on the mold's size. Do not simply follow the technical standards and hardness requirements in the design manual. The hardness standards in the manual are based on the heat treatment results of small specimens. When applying them to the actual product, it is important to determine an appropriate hardness index based on the actual situation. An unreasonable hardness index, such as one that is too high, will reduce the workpiece's toughness and cause cracking during use.

10. Is tempering color related to temperature?

After tempering, the surface of steel develops an oxide film called tempering color. In many cases, the tempering temperature can be determined based on the tempering color. Tempering color changes with temperature, so it can be used to roughly determine the tempering temperature. However, tempering color is also related to the tempering time, which is usually 5 minutes. The tempering colors of carbon steel at different temperatures, based on a 5-minute test, are as follows:

Light yellow: 200°C

Straw yellow: 220°C

Brown: 240°C

Purple: 260°C

Bluish purple: 280°C

Dark blue: 290°C

Blue: 300°C

Light blue: 320°C

Blue-gray: 350°C

Gray: 400°C

The tempering colors of stainless steel at different temperatures are as follows:

Light wheat yellow: 290°C

Wheat yellow: 340°C

Light reddish brown: 390°C

Light red: 450°C

Light blue: 530°C

Dark blue: 600°C

The tempering colors of low-alloy steel at different temperatures are as follows: Light wheat yellow: 225℃

Wheat yellow: 235℃

Light reddish brown: 265℃

Light red: 280℃

Light blue: 290℃

Dark blue: 315℃

However, many references only mention the relationship between color and temperature, ignoring the crucial consideration of time. At the same temperature, as the holding time increases, the final color will tend to be closer to the color at a higher temperature.