PRACTICAL APPLICATIONS OF THE CONCEPT OF SURFACE INTEGRITY

Transcription

1 PRACTICAL APPLICATIONS OF THE CONCEPT OF SURFACE INTEGRITY Antonín Kříž, Lukáš Kafka, Jiří Šimeček This paper is one of outcomes of the project no. FI-IM4/226, which is co-funded by the Ministry of Industry and Trade of the Czech Republic and the company HOFMEISTER s.r.o.

2 SURFACE INTEGRITY Surface integrity comprises a number of factors which must be not only measured but also understood. Surface integrity analysis only yields good results if all records are consonant (as in a symphony). 2/26

3 SURFACE INTEGRITY Surface integrity and all related factors being monitored are included in the ANSI B standard. The purpose of surface integrity analysis is to obtain a comprehensive description of the state of surface, as there are many factors affecting the resulting properties of a component. These factors operate simultaneously. They may be classified as internal and external ones. External factors: Mechanical (in-service stress) Chemical (corrosion) Physical (radiation, stray currents and others) Combination of multiple factors (stress corrosion, electrochemical corrosion and even manufacturing processes, such as chip removal, heat treatment, forming) Internal factors: Residual stresses Surface morphology (roughness) Material and mechanical properties of the surface (hardness, effects of hardening, state of microstructure, surface treatment, such as films and coatings) The presence of surface and sub-surface defects and heterogeneous microstructure (carbon in cast iron, inclusions, microporosity) 3/26

4 Different View of the Surface Integrity Geometric accuracy Surface roughness and profile Hardness Changes in microstructure Residual stresses Chemical and thermally-induced changes scaling, decarburization, carburization Mechanically and thermally induced cracks Changes in physical and chemical properties Understanding the surface requires that all the above aspects are considered in the appropriate context. 4/26

5 RESIDUAL STRESSES Residual stresses are in the centre of attention as they are induced during all manufacturing processes, particularly those employing heat and mechanical working. Metal cutting falls into this group of processes. The introduction of heat into the surface is non-uniform. This phenomenon alone forms plastic and elastic zones. Depending on the amount of heat, zones of plastic and elastic deformation develop. If additional mechanical factors are taken into account, these zones become even more significant. Upon cooling of the workpiece, the energy used for elastic deformation is not recovered. The deformation remains in the surface, resulting in residual stress. Residual stresses can be classified as either tensile or compressive stresses. The surface typically contains compressive stress but problems arise if tensile stress is present. The latter is often produced in the sub-surface layer. There are a number of other influences including elastic-plastic properties and the character of the stress. Rapid transition to tensile stress may produce undesirable conditions, possibly leading to sudden failure. Determining the character of stress and incorporating all other factors encountered in industrial applications is very difficult with existing measurement techniques. Residual stresses are measured by direct and indirect methods. 5/26

6 Measurement of Residual Stresses by X-ray Diffraction Analysis in Holes in C45 Steel The microstructure is heterogeneous Ferrite regions show more pronounced effects of strain Residual stress distribution along the circumference of a drilled hole Distribution of residual stresses in holes made by different processes turning, drilling 6/26

7 Practical Applications of the Concept of Surface Integrity Roller burnishing tool Amount of strain related to the machined surface Roller burnishing for the purpose of surface compression is an alternative to thermal, chemical and other surface compression processes (carburizing, nitriding, hard chromium plating, plating, diffusing, plasma spraying, electron-beam technology etc.). 7/26

8 Results for Surface Integrity of Drilled Holes with the Length of 3D and Tolerance Grade IT6-IT7 The project is aimed at tools for machining AA6082-T6 aluminium alloy, ČSN tool steel, ČSN plain carbon steel, ČSN cast iron and Inconel 718 nickel alloy. This disparate group of materials was selected intentionally, as the company intends to cover the entire range of drill bits for all its current and future customers. The presentation contains results achieved through the analysis of surface integrity of holes drilled in ČSN cast iron. 8/26

9 Experimental Conditions Holes drilled with three different tools of different designs have been investigated. The holes were marked A, B and C. Dimensions of cylinders to be drilled: d = 26 mm, l = 36 mm. After drilling 12 mm diameter holes, the cylinders were sectioned along their axis to allow analysis of the machined surface. The following tests were carried out: - Surface topography measurement in 2D and 3D; - Surface roughness measurement; - Surface microstructure observation; - Microhardness testing; - Microhardness nanoindentation testing; - Scratch testing; - Corrosion test; - Impact testing. Testing sample 9/26

10 The State of Surface of Drilled Holes Micrographs of the surface of individual holes were compared visually and by means of image analysis. The mechanism of extraction of surface particles in a drilled hole The surface state of the drilled element A B C 10/26

11 Tool Surface Extracted surface area [%] A B C /

12 Traces on the machined surface without any link to the machining conditions and the state of the tool A. The smallest extracted surface area was found in the element drilled with the tool A: 37% of the total measured area. At the same time, the greatest extracted area (about 57%) was found for the tool C. Using the drill bit B leads about 50% extracted surface. 12/26

13 The state of surface of the drilled holes in 3D Surface Tool A Surface Tool B 13/26 Surface Tool C

14 Visual evaluation of micrographs taken with the confocal microscope clearly shows that highest quality surface was produced by the tool A. This is mainly due to the fact that this machined surface has the smallest proportion of extracted area. This conclusion is evidenced by the following micrographs which show representative local profiles of the surface (length of 640 mm) for individual drilled holes. A surface profile of the hole drilled by the tool A B C 14/26

15 Surface Microstructure The surface machined with the tool A is more planar than those machined with tools B and C. One can also find spots where the surface is damaged by extraction of subsurface graphite. Surface Tool A Surface Tool B Surface Tool C 15/26

16 Strengthened thin surface layer tool A 16/26

17 Surface and Subsurface Microhardness Tool - drill Average value - surface Average value surface Average value - surface Average value - surface A B C Nanoindentation measurement of microhardness Indentation locations in individual regions Nanoindentation measurement was performed with the load of 250 mn and a dwell time of 12 s. Three elements from last cylinders drilled with the tools in question have been evaluated. The measurement was carried out in the base material, i.e. in the region unaffected by drilling (a). Another tested region was the machined surface (b). 17/26

18 It is the element drilled with the tool A which shows the highest nanoindentation-measured microhardness of all three evaluated samples in the machined region b. The average value is HIT=6,677±575 MPa. Using the tool B creates a strengthened surface with the average microhardness of HIT=6,528 ± 1,079 MPa. The average proportion of elastic strain for the tool B was the greatest of all tools: 27.2%. The lowest average proportion of elastic strain of 24.5 % was found in the case of the tool C. The average microhardness measured by nanoindentation for the tool C was HIT = 6,529 ± 1,079 MPa. The average increase in nanoindentation microhardness HIT from the region a to the region b was identical for B and C tools: 47%. For the tool A, this increase was 55% due to drilling and related plastic strain. There was the greatest amount of strengthening as well. 18/26

19 Results of nanoindentation measurement of HIT microhardness in individual regions 19/26

20 Surface Analysis by Scratch Testing A 3D image of a scratch channel taken using a confocal microscope The principle of scratch channel volume measurement 20/26

21 Tool A [mm ] A B C D Arithmetic Average ± Volumes of scratch channels on the machined surface of the cast iron show that the largest value (indicating the lowest surface hardness) was found in the element worked by the tool A. In contrast, the lowest scratch channel volume was found in the element drilled with the tool B. 21/26

22 Corrosion Resistance Tests The corrosion environment medium was 3% NaCl solution in distilled water. The ambient temperature during the test was C. Documentation was obtained using a stereo microscope in 5 minute intervals. The images were examined with the aid of LUCIA image analysis software and the results were plotted in charts. The most rapid corrosion was seen in specimens drilled with the tool B. The remaining drilled holes showed very similar results. Tool B 0,7 0,35 0,6 0,3 0,5 START 0,4 CENT ER END 0,3 0,2 0,1 zkorodovaná plocha zkorodovaná plocha Tool A 0,25 START 0,2 CENT ER END 0,15 0,1 0, čas [min] 22/ čas [min]

23 Testing of the Surface Resistance by Means of Impact Testing Specimens were subjected to identical loads consisting in 5,000 impacts with the lowest available energy. The purpose was high sensitivity measurement of properties in those surface areas where the effects of cutting tools were expected to differ. The dimensions of the craters did not vary significantly. It is due to the fact that the states of machined surfaces do not vary in such a manner which could be detected under the presently used impact test conditions. More detailed examination of dimensions and of the nature of defects in the crater revealed that machining with the tool C produced a more favourable type of surface than the use of the other tools. 23/26

24 Conclusions Tool A B C 1. Amount of extracted particles Non-uniformity of the surface Thickness of effectively strengthened layer Percentage of area with the strengthened layer Thermal effect on the surface during machining Not recorded 6. Changes during drilling a single hole Variations between the first (initial cut) and the last hole Results of corrosion testing Contact loading Impact test Sum of point scores (highest number best result) Rank 24/26

25 Obtaining reliable results requires that individual factors are treated with great attention. Fragmentary measurement, partial experiments and their results are not sufficient for evaluating final properties of a component. There is a number of experts engaged in research into this issue at the author s place of work and at the Department of Machining Technology of the University of West Bohemia. In spite of this, or even for this reason, the surface integrity concept offers both a great challenge and a risk, resulting from separation of individual results from the overall context. Embedding the surface integrity concept into real-world applications still is and will be a cumbersome task, not only due to issues related to transferability of the methodology but also with regard to measured values and their impact on the quality of the product. Another source of difficulties is the absence of a unifying theory which would enable comparison and quantification of individual factors of influence. Despite these problems, valuable findings have been obtained both from the academic viewpoint and in terms of practical application and evaluation of surfaces newly created by specially developed finishing" drills developed and manufactured by the company HOFMEISTER. On the basis of the above mentioned wide-ranging cooperation between the academic sphere and manufacturers and users of cutting tools, the findings and results building a comprehensive picture of the surface integrity concept will be further developed. 25/26

26 Thank you for your attention