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Effects of cryogenic cooling on surface layer characteristics produced by hard turning

Grzesik W., Żak K., Prażmowski M., Storch B., Pałka T.

Archives of Materials Science and Engineering

2012

Vol. 54, nr 1

6-12

Purpose: The purpose of this research is to test the applicability of cryogenic hard machining for improving surface integrity produced in turning operations on parts made of high-strength, low alloy 41Cr4 steel with hardness of 57š2 HRC. The aim of the research is to quantify the surface roughness and the mechanical properties of the sublayer produced under practical working conditions. Design/methodology/approach: The objectives were achieved by the characterization of machined surfaces using 2D and 3D scanning techniques. The surface profile and surface topographies were characterized and compared for optimal machining conditions. Moreover, microhardness beneath the surface was measured using a hardness tester with a Berkovich indenter. The microstucture of the sublayer was examined using SEM/EDS technique. Findings: This investigation confirms that hard machining allows producing surfaces with acceptable surface roughness and, in some cases, with attractive service properties. The main conclusion is that cryogenic hard cutting operations can partly eliminate grinding operations in cases when white layer is not produced. Research limitations/implications: The basic limitations concern the measurement of residual stresses and microstructural alterations including phase analysis. Another important problem is to optimize the surface integrity including surface roughness and sublayer properties. Practical implications: One practical outcome is selecting the machining conditions which guarantee the demanded surface finish together with bearing properties. Moreover, they should be selected in terms of desired microhardness distribution. Originality/value: Original value of the paper is the presentation of the effects of cryogenic pre-cooling of the workpiece in hard turning operations. Experiments were performed under the conditions combining low surface roughness with attractive service properties. This knowledge can support the design of technological processes of hard steel parts.

Thermodynamic optimization of cryogenic systems

Chorowski M.

Archives of Thermodynamics

2006

Vol. 27, no. 4

57-66

Superfluid He II cooling for high field superconducting magnets has been developed using the opportunity of large scale project in fusion power generation and high energy physics laboratories. The magnets are cryostated at temperature as low as 1.8 K. The necessary energy input for 1 W of cooling power at 1.8 K is of the order of 1 kW and the use of superfluid helium at the temperature below 2.0 K requires efficient and low energy consuming cryostats. The cryostat design can be optimized on the basis of the Second Law of Thermodynamics with use of entropy generation minimization method, within the boundary and technical constraints of the device. The paper presents the discussion of theoretical background of entropy generation minimization method and gives the example from the cryostat design practice. The basic scheme of superfluid pressurized He II cryostat is given.The He II cryostat built in the Institute of Power Engineering and Fluid Mechanics is described together with the optimization procedure of the cryostat thermal insulation system based on the entropy minimization method.