Real and Effective Heat Transfer Coefficients (HTCs) Used for Computer Simulation of Transient Nucleate Boiling Processes during Quenching

In the paper some unusual processes are considered during quenching such as self-regulated thermal process when metallic probe is covered by insulating polymeric layer, oscillation of temperature in surface layers of probe, creation a “shoulder” when quenching in polymer solution, possibility to perform austempering process just in cold liquids. Above mentioned processes build a basis for the new intensive quenching technologies and can bring a great benefit for heat treating industry when further carefully investigated. It is shown that initial temperature gradients, which cannot be governed by classical law of Fourier, can be tested by Liscic/Petrofer probe, etc. The paper discusses how organize such international investigation to satisfy contemporary practical needs and solve unsolved problems of science in the field of quenching. Also, the results of investigations can be used for software designing and cooling recipes development during quenching steel parts in liquid media. It makes a great progress because at preset time only cooling curves and cooling rates are available that are used for comparable purpose and cannot be used for recipes development.

Section: Materials Sciencementioning

confidence: 99%

Liscic/Petrofer Probe to Investigate Real Industrial Hardening Processes and Some Fundamentals During Quenching of Steel Parts in Liquid Media

Kobasko¹,

Liščić

2017

“…In this paragraph a difference between effective and real heat transfer coefficients (HTCs) is discussed. As known, the heat transfer coefficient during transient nucleate boiling process is evaluated as a ratio of the heat flux density produced by bubbles to the overheat of the boundary layer [11,12]…”

Section: The Main Differences Between Real and Effective Heat Transfementioning

confidence: 99%

“…That creates misunderstanding and leads to rather big errors during temperature fields calculations within the transient nucleate boiling processes due to big difference between real and effective HTCs (Table 1) [12,13]. When calculating data shown in Table 1, it was assumed that overheat at the beginning of transient nucleate boiling process was equal to 14 o C. As seen from Table 1, the ratio α real /α eff depends on temperature of a liquid.…”

Section: The Main Differences Between Real and Effective Heat Transfementioning

confidence: 99%

Study of Differences Between Real and Effective Heat Transfer Coefficients to Provide Correct Data on Temperature Field Calculations and Computer Simulations During Hardening of Steel

Kobasko¹

2018

The paper analyses contemporary methods and probes for testing liquid media used as a quenchant in heat treating industry. It is shown that lumped-heat-capacity method, often used for testing liquid media, produces big errors during transient nucleate boiling processes due to incorrect calculation condition Bi≤0.25 caused by use effective heat transfer coefficient (HTC). The effective heat transfer coefficients (HTCs), utilized for this purpose, are almost seven times less as compared with real HTCs that results in incorrect calculation the value of Bi. Instead of lumped-heat-capacity method, a general cooling rate equation is proposed for HTC calculation. It is underlined that effective HTCs can be used only for approximate core cooling rate and core cooling time of steel parts calculations. For investigation cooling capacity of liquid quenchants, including initial heat flux densities, HTCs and critical heat flux densities, high developed technique of solving inverse problem should be used based on accurate experimental data generated by testing liquid media with the Liscic/Petrofer probe or other similar technique.

“…1). Taking into account existence of polymeric layer, the author [3] proposed an equation for calculating initial heat flux density for any forms of steel parts which is covered with a thin layer of thickness δ. It can be written as:…”

Section: Initial Heat Flux Densities Evaluationmentioning

confidence: 99%

Designing of Advanced and Original Austempering Processes Based on Thermal Science and Engineering Physics Approaches

Kobasko¹

2016

In the paper, a small concentration of inverse solubility polymers in water and other liquid media is recommended to eliminate film boiling by means of reducing initial heat flux density. Quenching steel parts and tools in a small concentration of water solutions under pressure allows performing austempering process just using cold liquids. Its essence consists in coinciding martensite start temperature MS with the average temperature of self-regulated thermal process during nucleate boiling mode and further immediate transferring steel parts for tempering at the temperature which exceeds value MS. The new technology increases the service life of austempered workpieces by more than two times, saves alloy elements, is suitable for larger metallic components, improves environmental conditions, since instead of melted salts and alkali, plain water and water salt solutions can be used. The new austempering process can be used in forging shops to obtain super-strengthened materials in order to switch from alloy steel to plain carbon steels. And it can be also widely used for obtaining nano - bainitic structure in plain carbon steels resulting in saving alloy elements and improving mechanical characteristics of materials.