It is often encountered that hardness value of entrusted heat treatment product can only be at a certain value, and there should be no deviation! For example, if heat treatment hardness is required to reach 60HRC, if you reach 59HRC after heat treatment, or 61HRC, it will be regarded as an unqualified product. As everyone knows, allowable deviation of Rockwell hardness machine is still 1HRC. You explain heat treatment principle to him, and he will put on a face of God: Do you want to make my heat treatment product? Market competition! Heat treatment manufacturers have to bite bullet and take it. As for heat treatment manufacturers, how can they do it well? Colleagues can definitely guess it!
It's really "how bold people are, how productive land is."

Some people think that after quenching, it cannot enter tempering process before it has cooled to room temperature. In fact, many steel grades, especially low- and medium-carbon steels, have a martensite transformation ending point higher than room temperature. When they are cooled to room temperature, they are prone to cracking. After quenching, they can be transferred to tempering process as soon as possible.

This approach is not advisable, and furnace entry temperature before tempering after quenching should be determined according to martensite transformation point of steel grade! In order to prevent quenching and cracking, we can not speculate arbitrarily, adopt method of tempering with temperature in general!

Individual bosses claim to have secret to improve service life of mold! What is his secret? In the end, it turned out that after heat treatment is required to complete annealing treatment, quenching and tempering treatment cannot be performed immediately. Mold must be placed at room temperature for one week between annealing and quenching! Say yes: release annealing stress! I don’t know which expert can answer this truth? !

In order to save cost of product processing, some people process all dimensions before heat treatment, then go to heat treatment, quenching and tempering. Heat treater is required to ensure that there is no deformation during heat treatment process, or only allow amount of deformation to be within tolerance zone value of last cold working! Process of heat treatment is essentially a stage of structural deformation. Microscopic deformations accumulate. Who can guarantee that they will not show up as dimensional deformations macroscopically?

Many companies that commissioned products out-of-process have learned to request incoming inspections. Since leader put forward this request, guys treated it seriously, bought a Rockwell hardness tester and put it in factory. Products after heat treatment have begun to be inspected. These are beyond reproach, but they always fail heat treatment product inspection! This is too busy for heat treatment company, how could it be? It is clear that factory passed inspection, but how could it be disqualified in the hands of the user? Company is puzzled.
Heat treatment company takes it seriously and dispatches personnel to deal with matter urgently! You never know full extent of things until you see them! It turns out that they did not remove decarburization layer of heat-treated product (machining allowance is sufficient to ensure that there will be no decarburization layer remaining after processing), and directly hit HRC hardness on the surface of workpiece! How can this have high hardness?

Many materials show that iron-carbon equilibrium phase diagram is very important knowledge in heat treatment, it is basis for formulating heating process of steel materials, it is pointed out that especially heat treatment workers must be proficient in iron-carbon equilibrium phase diagram.
Iron-carbon phase diagram is structure diagram of iron-carbon alloy in equilibrium state, rather than transformation diagram of non-equilibrium martensite, bainite and other structures. Critical temperature parameters of iron-carbon phase diagram are limited to carbon steel and cast iron, non-alloy steel and alloy cast iron. Due to addition of other alloying elements, equilibrium diagrams of alloy steel and alloy cast iron are still very different from iron-carbon equilibrium diagrams.
Iron-carbon equilibrium phase diagram is result of extremely slow speed of heating and cooling, and it is limited to iron-carbon alloy steels. This theoretical state cannot be used in actual production. Structural transformation in heating and cooling process of heat treatment such as actual quenching is carried out at a certain heating rate and cooling rate, Therefore, iron-carbon equilibrium phase diagram is only a necessary basic knowledge and starting point for studying and learning heat treatment, rather than a phase diagram used directly in heat treatment process.
Knowledge of iron-carbon equilibrium phase diagram of heat treatment workers is only beginning of heat treatment learning, cannot reach realm of using iron-carbon equilibrium phase diagram to deal with actual problems of process.
A good iron-carbon phase diagram of heat treatment engineering is just one of introductory knowledge of heat treatment.

In annealing process of low carbon steel, many people think that equiaxed grains can be obtained. In fact, it is easy to obtain equiaxed grain size in boiling steel. It is difficult to achieve equiaxed grain structure in Al aluminum killed steel. Especially after annealing of cold-extruded deformed parts, crystal grains are obviously deformed and extruded! Even at an annealing temperature of 950℃ or higher, it is difficult to reach equiaxed grains.

People's direct thinking is: the lower hardness, the easier it is to squeeze and deform. In extrusion process of steel, spheroidized structure of pearlite has the highest deformability, but hardness of this structure is generally higher than that of flaky pearlite. Therefore, original structure of extruded part is technical requirement of pearlite spheroidized structure, and flaky pearlite structure with the lowest hardness cannot be used.

Among users who use hot forging dies, many people like to ask for high hardness, even 52-55HRC. This concept is wrong.
Reason for this phenomenon should be that some non-standard heat treatment companies or a certain "master" did not really quench forging die according to service conditions of forging die when doing external heat treatment business of forging die, but reduce quenching temperature. and shorten holding time to only meet user's hardness requirements. This hardness value seems to meet standard (or specification) forging die hardness range. Since red hardness is not considered, forging die has poor tempering resistance during use, hardness will soon decrease. When user inspects used forging die again, it is found that heat treatment hardness of forging die is not high. "Boss" of forging die used his brain: next heat treatment will increase hardness requirements, it turns out that forging die with increased hardness has a longer life than last forging die with hardness value selected in accordance with standards and specifications, so he is very happy: Increasing hardness can solve this problem. How could he know mystery that incompetent heat treatment level of heat treatment manufacturer or "master" caused hardness beyond standard and long life? As a result, this problem was misrepresented, causing hardness value of hot forging die to be higher and higher every day!
Hot forging die with red hardness within standard hardness range has a good life! It is incorrect for forging dies to require high hardness!

After solution and aging treatment of aluminum alloy parts, there are two methods for judging whether it is overfired during solution: metallographic method and surface state color method. According to color and state of surface of workpiece, it is convenient to judge whether it is overheated during heat treatment and solution treatment, but it requires rich experience. Metallographic method is accurate, but actual object must be dissected, which is a destructive test and judgment, which is easy to cause waste.
According to surface color and state of workpiece:
①Surface of piece is dark gray,
②There are small bubbles on the surface of workpiece,
③ Cracks appear, and crack fracture is rough.
In one of above situations, there is a possibility of over-burning. This is only observed on workpiece after heat treatment. When solution-aged parts have been processed and observed again, it is found that there are abnormal phenomena on the surface of aluminum alloy workpiece-roughness, deformation, wrinkles, etc., which cannot be simply regarded as heat treatment overburning. Since strength of aluminum alloy is still low compared with ferrous metals, it is necessary to analyze role and influence of subsequent process. Especially follow-up polishing and sandblasting treatment, impact on the surface cannot be ignored. When "water surface ripple" type wrinkles appear on the part of workpiece, it cannot be judged as heat treatment overburning, but sandblasting pressure is too high or sandblasting time is too long, which is cause of deformed layer formed on aluminum alloy surface. This "water surface ripple" type wrinkle does not have characteristics of aluminum alloy overburning, but has characteristics of plastic deformation on the surface under impact. At this time, it should be judged as: sandblasting defect!
Metallographic method was used to determine that it was a sandblasting defect.

Some people think that hardness selection in his design is selected according to hardness range in manual. Why can't you say that this hardness can't be achieved by heat treatment?
For example: spring steel 60Si2Mn is used to make large parts. Because actual thickness of workpiece is very large and thickness is obvious, heat treatment has no good way to achieve required hardness standard. Hardness in manual can reach: 58-60HRC. There is no way to combine actual artifacts. Can only reduce heat treatment requirements.
Hardness that determines heat treatment is controlled by following factors: material grade, mold size, workpiece weight, shape and structure, subsequent processing methods and other factors. After heat treatment of mold, internal and external hardness is not same. Material and design size should be selected according to size of mold. It cannot be selected directly according to technical standards and hardness requirements in design manual. Hardness standard in manual is derived from heat treatment results of small samples. When applied to real object, a reasonable hardness index must be determined according to actual situation. Unreasonable hardness indicators, such as too high hardness, will lose toughness of workpiece and cause cracks in the use of workpiece.

 

Many people who understand heat treatment think that heat treatment is difficult to learn, difficult to do, and it is not easy to grow actual talents. It is also said that heat treatment is to burn workpiece red and put it in water, and it will be fine. Is it that simple? Since it has become a discipline, it is certainly not that simple. If you look at all issues from perspective of those who "burn it red and put it in the water," then world will be fine.
When those people don’t need heat treatment, they always boast about how important heat treatment is and how people pay attention to heat treatment;
When he needs to entrust others to do heat treatment, he says that heat treatment is "burning red, just put it in the water", and he is unwilling to pay a more reasonable heat treatment processing fee;
When there are problems such as cracking, low service life, etc., it is considered that "heat treatment is the first of all evils" and it is fault of heat treatment;

If a company broke mold and injured operator during use of mold, company immediately notified heat treatment manufacturer: You injured someone during use of heat treatment mold in your home, how much do you have to compensate! Asked reason, answer was that this product was processed by your heat treatment, and there was an accident, so you are required to compensate.
Product failure should be analyzed in terms of design, material selection, material defects, process defects (including heat treatment), assembly and use, etc., to find out real cause. In order to shirk responsibility, it is unreasonable to arbitrarily determine that failure is caused by heat treatment. Why do doctors have to see patient in person? I think it is same reason that we must comprehensively analyze design, material selection, material defects, process defects (including heat treatment), assembly and use process of waste product for failure of product.
After incident was appraised by the most authoritative organization, quality of heat treatment was completely normal, and it was not cause of accident. Real reason is use problem ----- overload!
Lack of knowledge in a certain industry is desirable, but it is either a scientific attitude or ignorance to deal with problem.

This kind of statement is often encountered when dealing with heat treatment quality problems. After hearing this statement, heat treatment person is really dumbfounded. If you encounter such a customer, problem must lie with customer, not heat treatment problem! Because customers do not understand manufacturing quality process control before heat treatment at all, they did not consider creating a good pretreatment state for heat treatment.

Heat treatment must not only ensure qualified hardness value, but also pay attention to process selection and process control. Overheated quenching and tempering can achieve required hardness; similarly, if quenching is underheated, by adjusting tempering temperature, you can also make do with required hardness range. There are many people doing this. Some are under-heated quenching in order to save power consumption; some are under-heated quenching due to limit temperature limit of heating furnace. How does early failure of such heat-treated products have nothing to do with heat treatment?

Forging process is to eliminate material defects, improve structure and improve material properties, save amount of mechanical cutting and improve utilization rate of materials. However, today's forgers have forgotten "eliminating material defects and improving organizational morphology" completely, only "work hard" on ensuring forging size, completely disregarding requirements for improving material performance. What is even more amazing is that some materials through forging process, instead of improving performance of material, it ruins performance of material. Forgers indiscriminately use forging waste heat annealing method, resulting in formation of a serious network carbide structure in material.
Since heating temperature of material forging is mostly much higher than heating temperature of heat treatment and quenching, "serious network carbide structure" is inherited, which will bring serious consequences to product quality.

Statistics on causes of early failure of molds at home and abroad:

This data list shows that statistical results of past accidents are not applicable to prediction of future accidents. That is to say, in determining cause of a certain mold failure tomorrow, it cannot be considered that cause of mold failure is heat treatment accounting for 44 to 52%. Instead, we must make targeted analysis. This statistic has misled many people and formed a mindset that people think that mold failure is a heat treatment problem.

After tempering, surface of steel presents an oxide film color, which is called tempering color. In many cases, it is necessary to determine tempering temperature based on tempering color. Tempering color changes with temperature, so tempering temperature can be roughly determined based on tempering color. But tempering color is also related to tempering time, usually 5 minutes.
Tempering color of carbon steel at different temperatures is based on 5 minutes, and surface color is as follows:
Light yellow: 200℃
Grass yellow: 220℃
Brown: 240℃
Purple: 260℃
Blue-violet: 280℃
Dark blue: 290℃
Blue: 300℃
Light blue: 320℃
Blue-gray: 350℃
Gray: 400℃
Tempering color of stainless steel at different temperatures:
Light wheat yellow: 290℃
Wheat yellow: 340℃
Light reddish brown: 390℃
Light red: 450℃
Light blue: 530℃
Dark blue: 600℃
Tempering color of low alloy steel at different temperatures:
Lightwheat yellow: 225℃
Wheat yellow: 235℃
Light reddish brown: 265℃
Light red: 280℃
Light blue: 290℃
Dark blue: 315℃
However, in many materials, only relationship between color and temperature is mentioned, key premise of time is ignored. At same temperature, with extension of holding time, final color will be biased towards a higher temperature. It often causes misjudgment of actual temperature.

There are two concepts in heat treatment deformation: organization deformation, shape and structure deformation. Result of research is that when vacuum heat treatment obtains same structure and hardness compared with other furnace heat treatments, deformation is the smallest. That is: tissue deformation is minimal.
For shape and structure deformation, vacuum heat treatment is often not as small as other furnace types. Heat treatment of other furnace types, such as quenching, can easily control amount of deformation by grading, isothermal, and straightening outside furnace. Due to imperfections of these functions, vacuum quenching sometimes increases.
Confusion of these two concepts gives people impression that deformation of vacuum heat treatment is small, which is a wrong or incomplete understanding!

When analyzing carburization phenomenon of vacuum heat treatment workpieces, there are two misunderstandings: first, it is believed that workpiece is carburized in quenching oil; second, it is believed that graphite parts in heating chamber cause carburization. In fact, in many cases it is not these two reasons, but cleanliness of heating chamber is not high. A large amount of quenching oil is brought into heating chamber when workpiece enters and exits furnace, material basket is contaminated, feeding trolley enters and exits the hot chamber, remains on cold wall of heating chamber. When it is heated, a volatile reducing atmosphere is formed and workpiece is carbonized.
Except for directly entering oil at a temperature above 1050℃. When workpiece is heated under 1050℃ for oil quenching, a little pre-cooling into oil will not cause obvious carburization.
Carbon addition of graphite parts in heating chamber to workpiece cannot be ruled out, but there is no serious residual quenching atmosphere.
Carburization phenomenon of vacuum heating quenching is more serious because quenching oil pollutes furnace chamber, not reason why people say that quenching in oil or graphite parts are caused!