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

Nikolai Kobasko

doi:10.21303/2461-4262.2018.00646

Abstract

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 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.

Highlights

In this paper effective heat transfer coefficients (HTCs) and affective Kondrat' ev numbers Knef are considered and is shown possibility of their use for core cooling time and core cooling rate calculations of different steel parts [1,2,3]
A difference between effective and real HTCs is discussed in the paper
Such consideration is very important because historically in heat treating industry mainly effective HTCs are used for development recipes of cooling

Summary

Introduction

In this paper effective heat transfer coefficients (HTCs) and affective Kondrat' ev numbers Knef are considered and is shown possibility of their use for core cooling time and core cooling rate calculations of different steel parts [1,2,3]. Instead of the lumped-heat-capacity method, a universal cooling rate correlation of regular condition theory is used for evaluating effective Kondrat' ev numbers and HTCs. Real HTCs require exact measurement surface temperature of the probe during transient nucleate boiling process. 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], i. E. q αnb = Tsf − Ts. Here αnb is real HTC during transient nucleate boiling process; αef is effective HTC; q is heat flux density; Tsf is surface temperature; Ts is saturation temperature; Tm is bath temperature. Dimensionless correlations of authors [11, 14] were used for this purpose

Temperature of water solution

It means that

To T

Discussion

Conclusions