When hot metal gets plunged into a quenching medium, the microstructure freaks out a bit. Naturally, the metal wants to be at specific microstructures that vary across its temperature ranges. So, when the hot metal is suddenly in contact with the cold quenching medium, it tries to revert to the microstructure it finds natural at that temperature.
Full Answer
What happens to the microstructure of the test steel after quenching?
Figures 4 (b) and (c) show the microstructure of the test steel after quenching at 1040 and 1060°C, respectively. It can be seen that with the increase in solution temperature, the content and size of UC in the steel decreased gradually.
Does quenching and tempering affect the microstructure evolution and Bake hardening behavior?
The effects of quenching and tempering on the microstructure evolution and bake hardening (BH) behavior of both ferrite and dual phase steels were investigated. The C–Mn steels were heated to the soaking temperature, quenched in water and then tempered in the 100–500 °C range.
What happens when quench operation is interrupted?
Such residual stresses can deform workpieces or introduce dimensional incongruities. Likewise, if the quench operation is somehow interrupted, the grain boundaries grow fuzzy. That’s something of a problem, as fatigue resistance problems propagate when the crystal matrix doesn’t form properly.
What are the advantages of quenching a material?
In some cases, quenching can make a material more ductile, and softer. In the case of most steels, you saw that the speed of quenching allowed you to get a metastable martensite, instead of the thermodynamically favored ferrite. However, in other materials (or some unique steels), quenching can let you avoid a strengthening or embrittling phase.
What happens to microstructure after quenching?
Quenching leads to the highest hardness values, as expected. This is due to the formation of huge amounts of martensite, from austenite, as observed in the microstructure. Tempering after heat treatment of EN 8 steel always leads to a reduction in hardness.
What does quenching and tempering do to microstructure?
The microstructures after quenching and tempering treatment comprise mainly martensite. As the tempering temperature increases, the martensite laths gradually recover. Figure 10(d) shows that the carbides precipitate preferentially along the martensite lath boundaries after being tempered at 570 °C.
What happened to the grain structure after quenching process?
After the metal is heated, we need to rapidly cool the steel. As material is quenched, the austenitic grain structure created during the heating stage transforms to different grain structures such as martensite, ferrite, pearlite, and cementite.
How does heat treatment change microstructure?
1:403:32Effects of Heat Treatment on the Microstructure and Mechanical ...YouTubeStart of suggested clipEnd of suggested clipWhen the specimen is subjected to an ailing process the ferrite grains undergo completeMoreWhen the specimen is subjected to an ailing process the ferrite grains undergo complete recrystallization due to the slow cooling in annealing austenite is transformed to soft pearlite.
What happens to the microstructure during tempering?
Changes in Martensite Structure The martensite itself also changes during tempering. One change that occurs is that the “tetragonality” of the martensite is reduced as the martensite becomes more and more like cubic ferrite as the carbon leaves the martensite.
What happens to the microstructure of steel during tempering?
During tempering, the particles coarsen and become large enough to crack, thus providing crack nuclei which may then propagate into the matrix. As a consequence, untempered low--carbon martensitic steels sometimes have a better toughness than when they are tempered, even though the untempered steel is stronger.
How does heat treatment affect grain structure?
Grain Size Effect: It has long been known that the properties of some metals could be changed by heat treating. Grains in metals tend to grow larger as the metal is heated. A grain can grow larger by atoms migrating from another grain that may eventually disappear.
Does heat treatment change grain structure?
As the temperature rises in a heat treatment furnace, strange transformations occur. In steel workpieces, the grain structure of the component changes size. Alternatively, new grains form. They undergo phase transformative processes.
How does quenching affect grain size?
Grains are separated by boundaries called dendrites. Grain size greatly affects the toughness of steel. Smaller grain size gives tougher or harder steel. When steel is heated and cooled quickly, as in quenching, smaller grain size is the result.
What happens to the microstructure during annealing?
Annealing is a heat treatment process which alters the microstructure of a material to change its mechanical or electrical properties. Typically, in steels, annealing is used to reduce hardness, increase ductility and help eliminate internal stresses.
What happens to the crystal structure in the metal when it is heat treated?
The FCC transformation occurs when steel is heated above its critical temperature. The bonds between iron atoms are relaxed from their BCC state, and transformed into the FCC structure. The important thing to note is the effect of the increased atoms in the lattice.
How does annealing affect microstructure?
Annealing process can improve material structure and reduce microstructure defects. And it can decrease hardness and improve the cutting machining properties of metal. Diffusion annealing can make the composition of the material uniform and reduce the anisotropy of the material.
Abstract
The effects of quenching and tempering on the microstructure evolution and bake hardening (BH) behavior of both ferrite and dual phase steels were investigated. The C–Mn steels were heated to the soaking temperature, quenched in water and then tempered in the 100–500 °C range.
1. Introduction
Recently, there has been a growing interest in the dual phase (DP) steel due to the combination of continuous yielding, low yield strength, high tensile strength, good ductility and high bake hardening value [1], [2], [3].
2. Experimental procedure
Cold rolled low carbon steel with chemical composition (in wt%) of 0.083C, 0.033Si, 0.322Mn, 0.021 P, 0.006S was used in this research. After reheating to 1200 °C for 1 h and hot rolling to 3 mm, the steels were coiled at 720 °C, and then cold rolled to 0.4 mm thickness.
3. Results and discussion
Fig. 2 shows the tendency of the BH value with the quenching temperature. As seen in Fig. 2, with an increase in the quenching temperature, the BH value decreases for the steels without tempering. For example, the BH values are 46 MPa and 52 MPa for the steels annealed at 800 °C and 900 °C, respectively, at the quenching temperature of 600 °C.
4. Conclusions
The residual stresses, carbon content and free dislocations density play an important role in the bake hardening behavior in the DP steel. Increasing the annealing temperature results in higher BH values due to the dissolution of carbides.
Acknowledgments
This work was supported by a grant from the National Key Project of Scientific and Technical Supporting Programs of China (No. 2011BAE13B07 ).
Why is water used as a quenching medium?
For this reason, it is a widely used quenching medium. Water cools metal far quicker than oil does. As with oil, you could submerge the part in question in a water bath and leave it there , or circulate water to flow over or through the piece.
What is quenching in metal?
Quenching Definition. Quenching is defined as the rapid cooling of a material, usually metal, in a quenching medium to obtain specific material properties. The quenching medium is often water, brine, air, or oil. In metallurgy, quenching forms part of the hardening process, rapidly cooling steel from high temperatures to obtain martensitic ...
What happens when a hot metal is in contact with a cold quenching medium?
So, when the hot metal is suddenly in contact with the cold quenching medium, it tries to revert to the microstructure it finds natural at that temperature. Unfortunately for the metal, the cooling process is rapid, so the microstructure can’t do this conversion fast enough.
Why does a metal quenching medium boil?
In other quenching media, air bubbles form on the metal’s surface as it heats the quenching medium, causing it to boil (local evaporation). These air pockets inhibit cooling since air does not conduct heat as efficiently as water or oil.
What is brine cooling?
Brine, a solution of water and salt, is a highly effective cooling medium. As with water and oil, you could use a stationary quenching bath or circulate brine over the part in question to enhance the quenching speed.
What is the most common quenching medium?
There are various types of quenching media available in metallurgy. The most commonly used quenching media are water, brine, oil, and air. It is essential to match the quenching medium and its temperature to the metal in question, as well as the starting temperature.
Is quenching medium cold enough?
For example, a screw hole very close to the edges of the piece. Quenching could also be inadequate. If the quenching medium isn’t cold enough , we will see gradual cooling, negating all the benefits gained through the heating process.
What can quenching do in steel?
The first thing that quenching can do is to refine grains. If you didn’t know, metals are crystals. Unlike the crystals you might normally think of, like gemstones, metals are “polycrystalline.”.
What is the purpose of quenching?
Quenching is done by heating the metal and rapidly cooling it in a quenching medium such as water or oil. Proper quenching can precisely control the final microstructure and phases present in the alloy.
What phase of steel is quenched?
When quenching steel, we first need to heat it past the austenitizing temperature. In hypoeutectoid steel (left of the eutectic point) the steel will have the austenite phase. In hypereutectoid steel (right of the eutectic point) the steel will have two phases: austenite and cementite.
What is the process of rapidly cooling a material?
Quenching is the process of rapidly cooling a material (usually a metal) in order to obtain desirable mechanical properties like increased strength and hardness. Most people think quenching is just dunking red-hot steel into a bucket of water, but materials scientists can quench in water, oil, liquid nitrogen, or even air.
What is the fastest quenching media?
The main way to control the cooling rate is by using a different quenching media. Salt water is usually the fastest practical quenching media. Liquid nitrogen is a relatively slow quenching media because of its low thermal conductivity and specific heat. If the alloy is cooled too rapidly it may crack.
What media is used for quenching?
The most common quenching media are water, brine (salt water), oil, liquid nitrogen, and air. Each of these media has different advantages and disadvantages.
Why is water a fast quench?
Water is one of the most common quenching media, because it’s easy to get and results in a fast quench. Water isn’t flammable and has a large specific heat and heat of vaporization, so it cools the material quickly when the water boils. However, the bubbles from boiling reduces the thermal conductivity, ultimately leading to a slower quench. (This layer of insulation caused by gas bubbles is called the Leidenfrost effect).
Why is steel considered a quenching material?
Steel deserves a special mention when the quenching process is being discussed because its mechanical properties are very sensitive to quenching. Through a quenching process known as quench hardening, steel is raised to a temperature above its recrystallization temperature and rapidly cooled via the quenching process.
What is quenching metal?
What is Quenching? Quenching is a type of metal heat treatment process. Quenching involves the rapid cooling of a metal to adjust the mechanical properties of its original state. To perform the quenching process, a metal is heated to a temperature greater than that of normal conditions, typically somewhere above its recrystallization temperature ...
What happens when you quench steel?
The rapid quenching changes the crystal structure of the steel, compared with a slow cooling. Depending on the carbon content and alloying elements of the steel, it can get left with a harder, more brittle microstructure, such as martensite or bainite, when it undergoes the quench hardening process.
What is the best way to quench metals?
To quench with oil, a heated part is lowered into a tank that is filled with some type of oil. The oil can also be flushed through the part.
Why are steels annealed?
For this reason, some steels are annealed or normalized following the quench hardening process.
Why is air quenching important?
Air quenching is also more intentionally performed when it is compressed and forced around the metal being quenched. This cools the part more rapidly than still air, although even compressed air may still cool many metals too slowly to alter the mechanical properties.
What is the purpose of air in metal quenching?
Air. Air is a popular quenching media used to cool metals for quenching. Affordability is one of the main benefits of air; its affordability is a result of its profusion on earth.