RESIDUAL STRESSES DUE TO WATER JET CUTTING

N. Ganev, I. Kraus, G. Gosmanová

Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehova 7, 115 19 Prague 1, Czech Republic

The type and magnitude of residual stresses generally depend on the material treated and on the parameters of the cutting technology applied. Qualitative and quantitative characteristics of the surface state of residual stress can play an important role in the service life of working pieces. Compressive stresses increase the fatigue limit and wear resistance, whereas tensile stresses decrease the fatigue strength and make destruction of frictional surfaces easy.

Recently "near-net-shape" technologies of machining have been of increasing interest. From this point of view processes of powder metallurgy and technologies of precise forging and casting are important because of the minimum of both the roughing and finishing operations using high-speed machining.

The unconventional "Water Jet Machining" (WJM) is one of the most suitable (progressive) technologies for materials difficult to machine and working pieces of complicated shapes. At the same time, heat loading of the surface layers of the cut materials is minimal.

Material cutting based on the abrasive effect of high-pressure water jet (WJM) has been in widespread use both in mechanical engineering and many other areas of industry. There are two basic modifications of this method: cutting by "pure" water jet (Waterknife) and cutting by using water jet with abrasive admixture (Paser-Particle Stream Erosion). Materials like SiC, Al2O3, diamond powder, ground garnet, quartz beads are usually used as abrasive substances. Automation of this cutting technology (still performed predominantly empirically) requires collecting enough information on the relationship between the parameters of the technology and both physical and mechanical properties of surface layers after cutting. The state of residual stresses is among the data that are not reliably known.

Samples investigated: Samples of steel _SN 11 373 prismatical in shape 5´25´15 mm3. The reference sample S (being of the same dimensions) was annealed for 2 hours at 650°C and then a surface layer 0.1mm thick was electrolytically etched.

The WJM technology used: Water pressure p = 250MPa, abrasive material beads (0.12¸0.35)mm in diameter; cut AWJ1 was performed by ground garnet with flow rate of 630g.min-1; cut AWJ2 - quartz sand, flow rate 560g.min-1; cut AWJ3- quartz sand, flow rate 240g.min-1.

The diffraction technique used: X-ray residual stress measuring method "sin2y" was performed by means of an w­goniometer SIEMENS with CrKa radiation. Diffraction line (211) was detected within the interval 2q = 149° ¸ 162° with the step scan 0.2° and constant time (10s) exposition. Calculation of surface stresses and their gradients within the penetration depth of the applied radiation was based on the shift of the centroid of the diffraction line and X-ray elastic constants 1/2s2 = 5.76.10-6 MPa-1, ­s1 = 1.25.10-6 MPa-1. The area of 45mm2 was irradiated at y = 0°, and 75mm2 at y = 63.43°.

Measurement results: Distributions 2q(sin2y) obtained from the surfaces AWJ1, AWJ2 and AWJ3 were used for residual stress evaluation when the state of stress is assumed to be: - biaxial isotropic in a plane parallel to the surface (Table 1), - three-axial inhomogeneous, isotropic in a plane parallel to the surface (Table 2). The calculations were performed by using the stress-free value 2q0 = 156.41° obtained from the reference sample S.

Table 1 Average residual stresses sj and sums of principal stresses s1 + s2.

Sample sj, MPa s1 + s2, MPa 
AWJ1
 
-186 ± 6
 
-434 ± 13
 
AWJ2
 
-276 ± 3
 
-612 ± 6
 
AWJ3
 
-336 ± 5
 
-740 ± 10
 
S
 
-25 ± 7
 
-46 ± 15
 
Table 2 Values of surface residual stresses s(0) and gradients g11, g33.

Sample
 
s(0), MPa
 
g11, MPa.mmm-1
 
g33, MPa.mmm-1
 
AWJ1
 
-117.5 ± 16.4
 
-7.7 ± 5.2
 
6.9 ± 1.2
 
AWJ2
 
-235.9 ± 6.9
 
1.7 ± 2.2
 
8.5 ± 0.5
 
AWJ3
 
-263.7 ± 3.6
 
-7.0 ± 1.1
 
8.0 ± 0.3

Conclusion: The technologies of abrasive water jet cutting used give rise to a compressive isotropic state of residual stresses. The magnitude of stresses depends on the type of abrasive material used and on its flow rate.

 

  1. Kraus I. and Ganev N.: X-ray Diffraction Analysis of Non-uniform Stress Fields, Acta Polytechnica, Vol.38, No.1, 1998, 25-62
  2. Kraus I. and Ganev N.: Diffraction Analysis of Mechanical Stresses, Czech TU in Prague, Prague 1995

 

This research is a part of the research project supported by the Grant Agency of the Czech Republic (Grant 101/96/1181)