FIGURE 3. Schematic diagram of specimen morphology after accelerated and decelerated laser quenching. It can be seen from the aforementioned figure that the variable velocity laser quenching process shows little difference in morphology, and the quenching area and boundary area of the material surface after laser quenching still maintain a high surface quality.

On the side view of the uncut section, it can be seen that the quenching layer at the edge of the specimen is also consistent with the variation of the scanning velocity. It can be seen that the quenching scheme of automatic control of scanning velocity by the laser quenching system can meet the process requirements of variable depth and surface hardness.

In addition, because the surface of the specimen is not protected by inert gas during the quenching process, the morphology of the intermediate variable velocity region of the laser quenching of the specimen shows a certain amount of spotted oxidation zones, which may be due to the unstable quenching velocity.

After completing all the experiment cases, we performed Rockwell hardness experiments on the surface of specimens quenched by a variable velocity laser. The specimen was then subjected to a non-destructive longitudinal cutting to measure its actual quenching depth. The measurement results of the second group of specimens are averaged, and the distribution results are interpolated by multiple term functions to obtain the fitting results, as shown in Figure 4.

FIGURE 4. A Deceleration quenching results. B Accelerated quenching results. Results of variable quenching scanning velocity. As it is shown in Figure 4A , during the scanning process of deceleration laser quenching, the actual quenching depth of the specimen is basically consistent with the reference specimen, and the average hardness is basically consistent with the reference specimen in the initial stage, but it tends to the maximum value and remains stable in the final stage of deceleration.

It can be seen from Figure 4B that during the scanning process of decelerated laser quenching, the actual quenching depth of the specimen is basically consistent with that of the reference specimen, while the hardness is basically consistent with the result of the reference specimen in the initial stage of acceleration.

However, in the final stage of acceleration, there is a slight change and its value decreases slightly faster. From the previous numerical analysis, it can be seen that whether it is accelerated or decelerated scanning; the effect of laser quenching on the surface Rockwell hardness of 45 steel is consistent with that of constant velocity scanning.

The reason is that despite the high frequency and high-power laser quenching used in the experiment, the energy transfer efficiency is sufficient to meet the energy demand of the phase change of the material surface structure in a short time, but the increase of hardness depends on the specific composition of the structure, especially the carbon content level.

However, during variable speed quenching in a short time, the accumulative effect of laser energy on the surface of the material and the temperature rise of the shallow layer of the material are different, resulting in a slight difference in the final quenching performance. Then, we numerically analyzed the actual output power of the dimensionless laser and the average quenching depth of the variable velocity scanning quenching specimen, ignoring the effect of laser scanning time.

The results are shown in Table 2. According to Table 2 , when the influence of the heat accumulation effect caused by scanning time on the quenching depth is not considered that is, when the influence of the scanning velocity is not considered , the laser source power in the laser quenching process is almost proportional to the actual quenching depth because the higher the laser output power in unit time, the more heat the material receives, resulting in a rapid temperature rise and faster radiation to the deep material.

Although the output power and scanning velocity of the laser source have a major impact on the actual effect of quenching, the program control adjustment of the scanning velocity can change the quenching performance within a certain range when the laser power is inconvenient to change in production applications.

Coincidentally, for a commonly used small gear with the modulus about 5—10, the laser scanning velocity adjustment range can be applied to their tooth surface quenching depth changes.

However, according to [ 17 ] and Equations 4 , 8 , the maximum hardness of metal materials achieved by laser quenching is related to the material itself. Therefore, in practical applications, when the same material is quenched to a specified depth using variable parameter laser parameters, its final performance can be determined by dimensionless laser scanning speed and power.

Variable velocity laser quenching is one of the most effective methods to precisely control the variable depth quenching process of the gear tooth surface, compared with other quenching techniques and control methods.

Although laser quenching has excellent consistency and controllability compared to other quenching techniques, we do not recommend changing the scanning velocity too fast to avoid other uncontrollable damage on the surface of the specimen during laser quenching without inert gas protection.

For further summary, we have the following conclusions. The reason is that the microstructure characteristics of the alloy at a given quenching depth, especially the carbon content, determine its maximum hardness level. Compared to the results of material properties under two experiment conditions, it is found that the rate characteristics of heat accumulation effect deceleration quenching may be more suitable for laser quenching of the gear surface with increased laser quenching depth.

DR proposed the idea of this paper, finished the formulation and implementation of the research content and experiment program, and then completed the main writing of the manuscript.

PZ investigated and sorted out the application of laser equipment and the related literature studied in this paper, and undertook the manuscript collation and submission process. JY carried out the experimental implementation process and operated the experiment data collection, screening, and statistical analysis in this paper.

YY analyzed the mechanism and application background of laser quenching for strengthening the mechanical properties of alloy materials.

In addition, YY provided a correction for the research direction and technical route of this paper. This study was funded by the Pre-Researching Key Project of National Defense: The authors thank Dongguan Huawei Laser Equipment Co.

The authors declare that this study received contribution from Huawei Laser Equipment Co. The company was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers.

Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Scientific Rep 11 1 — Cai S, Sun J, He Q, Shi T, Wang D, Si J, et al. Eng Fail Anal The intention of quenching is to transform the austenitic phase into martensite, which is an exceptionally hard phase of steel.

To achieve this, the material is exposed to a cooling media and the cooling time is reduces. When heating hypoeutectic steels over Ac1 and not Ac3, some ferrite will still remain in the structure after quenching, this reducess hardness.

For hypereutectic steels, it is optimal to heat the steel to a temperature between the Ac1 and Acm phase limits and thus, secondary cementite will remain in the structure next to martensite.

When going over Acm, there is the risk of the formation of a coarse-grained structure after quenching which makes the steel very brittle.

Overall, the quenching process leads to increased hardness and reduced ductility of the material. But the brittleness also increases and thus, to avoid unwanted side effects such as cracking and distortion, choosing a suitable cooling rate is essential. Quenching and tempering processes are often used in sequence to restore some of the lost ductility and reduce hardness to suitable levels.

Tempering also reduces brittleness. Annealing is also occasionally performed to reduce the hardness of quenched steel. Quenching is used when increased hardness is a requirement. Many applications such as construction, mining, heavy machinery, military, etc. require metals with a hard surface.

One that can resist abrasion, scratches and take impacts. The increased hardness from quenching is capable of providing these qualities. Quenching can be done for a wide range of materials but steel is the most commonly quenched metal.

Quenched steel shows extreme hardness. This method has a higher heat transfer rate than bath quenching. The bath quenching process is, however, more common. In bath quenching, the material to be cooled is placed in a bath of liquid or gas. As the quenchant surrounds the material, it is rapidly cooled.

But even this rapid cooling occurs at different rates from the time quenching begins until it ends. Let us look at these different stages in the next section.

During the bath quenching process, the material undergoes three distinct cooling stages. These stages are:. As soon as the material is placed in the quenching media, the vapour stage begins. Since the temperature of the hot material is above the boiling point of the quenching media, the media turns to vapour and thus, a stable vapour blanket is formed around the material.

Proper agitation of the quenchant expedites the cooling process causing it to enter the boiling stage. In the boiling stage, the vapour blanket stops forming.

The surrounding liquid takes its place and the process keeps on repeating. Among the three quenching stages, this stage has the highest heat transfer rate.

The third stage is the convection stage. However, it absorbs heat from the metal and rises. Surrounding liquid takes its place and a convection process is set in place. Formula 1 takes into account the grain growth index n, and formula 2 takes into account the time index m. Both equations are based on certain experimental data [9] Taking into account the grain growth index n and the time index m, a more reasonable model of the grain growth law can be obtained, as shown in Equation 3 :.

There are four unknowns m, n, A and Q in Equation 4 , which can not be determined directly by linear regression. When n sets different values, the error values of M, Q, and A are calculated, respectively.

The sum of the squares of the errors is a function of n, and the goal of optimizing the sum of the regression errors is the minimum. By the method of function fitting, the relationship between the error square and the change of n value is obtained.

Therefore, under isothermal conditions, the austenitic grain growth model of Q steel is:. Figure 5 is a comparison of austenitic grain size measurements and calculated values at different temperatures for insulation time 30 min.

The explanatory formula 7 can objectively reflect the isothermal growth law of austenite grains and has important reference value. The above grain growth model can only describe the average size changes of grains, but in fact, the growth of grains may have serious heterogeneity during the growth of grains, and a few grains will grow up abnormally.

This is because during the heating process, after the austenite transformation occurs, the grain boundary migrates under the driving force of reducing the free energy of the interface, resulting in the growth of the grain. Figure 3. Figure 4. Figure 5. Calculated and measured average size of austenite grains.

It can be seen from Figure 6 that when the quenching heating temperature is ˚C, due to the low austenitizing temperature, the amount of carbon and alloying elements dissolved into austenite is relatively small, reducing the hardenability of steel [13] [14] , In the subsequent cooling process, there was a certain amount of ferrite and bainite formation, and the proportion of martensite tissue was relatively small.

At the same time, because the original austenite grain is small, the resulting martensite strip beam is short and narrow, and the directionality is not obvious.

When the quenching heating temperature reaches ˚C,. Figure 6. Tissue pictures of Q heat treatment at different austenitic temperature for insulation times 30 min. the proportion of martensite in the tissue increases significantly. When the temperature continues to rise to ˚C, due to the coarsenization of some austenite grains and the full dissolution of alloy elements, The resulting martensite strip beam becomes wider and longer, and the strip beam is regular and directional.

When the heating temperature rises to ˚C, with the increase of quenching temperature, the size of austenite grains increases, and the size of the strip beam obtained after quenching increases accordingly [15]. The austenite grain and martensite strip bundles are severely coarse, the grain boundary is clear, and the austenite grains are divided into several parts by different orientation martensite strips.

From this it appears that although the chemical composition of the steel has not changed, However, the different quenching temperature has important influence on the microstructure of Q steel.

Therefore, it will lead to great differences in mechanical properties. As can be seen from Figure 7 , when quenching at ˚C - ˚C, with the increase of austenitizing temperature, the hardness and strength of Q steel gradually increase, reaching a maximum of ˚C; At ˚C, the maximum value was reached, of which the Rockwell hardness was 46HRC and the yield strength was MPa; During quenching at ˚C - ˚C, the hardness and strength of Q steel gradually decreased with the increase of austenitizing temperature.

Figure 7. Strength and hardness of Q steel at different austenitic temperatures after 30 min insulation. heating temperature, the amount of carbide dissolved into the austenite by the alloy element increases, the alloying degree in the austenite increases, and the carbon and alloy elements in the martensite increase after quenching; Moreover, at higher heating temperature, due to the dissolution of carbon and alloy elements, the stability of the overcooled austenite is increased, the hardenability of the steel is enhanced, and the martensite tissue content of the hardened steel is increased.

Therefore, the martensite obtained by quenching at a higher austenite temperature has higher hardness and strength. During quenching at ˚C - ˚C, with the increase of heating temperature, the austenite grain and martensite strip bundles were significantly coarsely coarser, so the hardness and strength of the steel decreased with the increase of heating temperature.

When the temperature is lower than ˚C, the grain grows less obvious. This causes the rapid coarsening of austenite grains. At different heating temperatures and thermal insulation times, the austenitic grain growth model of Q steel is:. where d is the average diameter of the final grain μm ; d 0 is the average diameter of the initial grain μm ; t is the heat preservation time min ; T is the heating temperature K ; R is the molar gas constant.

During quenching at ˚C - ˚C, with the increase of austenitic temperature, the hardness and strength of Q steel gradually increased, and the hardness and strength reached a maximum at ˚C. During quenching at ˚C - ˚C, the hardness and strength of Q steel gradually decreased with the increase of austenitic temperature.

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Both instances require speed. Metallurgists need to rapidly drop the temperature of a forged piece to achieve specific material properties, while firefighters work to stop destruction of property as quickly as possible.

Quenching with water is only effective beneath a critical temperature —any higher and the water levitates on its own vapor and can no longer boil the heat away. Boreyko's team wanted to see if using ice, rather than water, could bypass the levitation issue to enable the quenching of ultra-hot surfaces.

To conduct this research, Edalatpour and Colón heated an aluminum stage and measured the cooling rate of water versus ice. To ensure a direct comparison, they released the same amount of water and ice onto the surface after it was heated to a desired temperature.

When the initial surface temperature of the stage was between °C and °C, both the water and the ice successfully quenched the surface below °C.

The ice, however, achieved that result in half the time. At higher initial temperatures—°C to °C—only quenching with ice was successful. Heat transfer with ice was more than times more effective than with liquid water at these high temperatures.

What was the difference? The properties of water prevent it from hitting the sweet spot for removing heat. That sweet spot is boiling, because the steam escaping in bubbles most efficiently carries the heat away.

Because water easily levitates on its vapor at high temperatures, it becomes insulated from the surface and the boiling never occurs. Ice behaves differently. When dropped onto a hot surface, ice absorbs much of the heat as it melts.

This reduces the amount of heat available for producing vapor bubbles, preventing the levitation problem. In other words, the meltwater boils at a slower pace compared to pure water, thus helping to maintain boiling at high temperatures.

It turns out that water is the same way when exposed to ultra-high temperature surfaces: It is so focused and productive at boiling water into vapor that it experiences 'burnout,' which is the scientific term for levitation and the catastrophic failure in cooling that results.

So ice is like the slow and steady tortoise that wins in the long run. It doesn't make vapor bubbles very well, but this allows it to keep boiling and avoid levitation when things get heated.

The group's hypothesis of using ice for quenching followed its recent discovery that ice does not levitate and lose its boiling capability until °C, compared to °C for water. Based on those findings, Boreyko's team began several new projects applying its principles.

This heat transfer is the first outgrowth to be published. Colón's follow-up work includes measuring the cooling performance of ice when the surface is fixed at a constant temperature rather than being allowed to cool down.

There's a lot more to figure out before this becomes an on-the-shelf technology. More information: Jonathan R.

Boreyko, Ice quenching for sustained nucleate boiling at large superheats, Chem DOI: Provided by Virginia Tech. More from Chemistry. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page. For general inquiries, please use our contact form.

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April 14, Editors' notes. Editors have highlighted the following attributes while ensuring the content's credibility: fact-checked peer-reviewed publication trusted source proofread. Camryn Colón sets up an experiment in Associate Professor Jonathan Boreyko's lab. Credit: Photo by Alex Parrish for Virginia Tech.