Jet Clean Systems AG

Surface preparation for NDT

Jet Clean Systems

Cleaning Technology

© 2016 Jet Clean Systems AG page ii of 11

issued: 16.01.2015

printed: 07.10.2016

Table of content

1. SUMMARY .......................................................................................................................................................... 3

2. INVESTIGATIONS ................................................................................................................................................. 3

2.1 INTRODUCTION ........................................................................................................................................................ 3

2.2 SURFACE QUALITY .................................................................................................................................................... 3

2.2.1 Comparison of the surface quality after dry blasting and Suspension technology cleaning on stud bolts . 3

2.2.2 Surface quality: Application of Suspension technology cleaning with different abrasives ......................... 6

2.3 SURFACE DEFORMATION............................................................................................................................................ 7

2.3.1 Lateral polished section .............................................................................................................................. 7

2.4 BLASTING RESIDUES .................................................................................................................................................. 8

2.4.1 Scanning electron microscopy ..................................................................................................................... 8

2.5 LUTING OF CRACKS ................................................................................................................................................... 9

2.5.1 Tests using dry blasting ............................................................................................................................... 9

2.5.2 Tests using Suspension Technology cleaning ............................................................................................ 10

3. REFERENCES .......................................................................................................................................................11

3.1 INTERNATIONAL STANDARD ...................................................................................................................................... 11

3.2 EXTERNAL REPORTS ................................................................................................................................................ 11

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1. Summary

This report investigates different aspects of surface preparation important for non-destructive testing (NDT).

It shows clear influences of different blasting methods on NDT results. Summarized it can be stated that all

methods used influence the NDT results. The use of conventional dry blasting methods affects the detect-

ability of cracks bye dye penetrant heavily, with the worst results seen when using glass beads.

2. Investigations

2.1 Introduction

All norms1 for non-destructive testing (NDT) specify qualitative the need for surface preparation before exe-

cuting the testing. As there are no quantitative values specified for the different methods regarding surface

quality, cleanliness or other relevant aspects several tests were executed by using Suspension Technology

and different conventional dry blasting methods.

2.2 Surface quality

To compare the cleaning methods regarding the surface finish, dry blasting has been compared with clean-

ing by Suspension Technology. Two test series have been carried out. The first test has been made with

bolts and the second test has been made with ground specimens to relate the cleaning results to a very

smooth initial surface.

2.2.1 Comparison of the surface quality after dry blasting and Suspension technology cleaning on

stud bolts

The tests2 have been carried out on stud bolts. In a first step, six specimens have been dry blasted with dif-

ferent abrasives:

a) Garnet mesh 120 b) Garnet mesh 200 c) Corundum d) Glass beads e) Cleaning with Garnet mesh 120 and surface finish with Corundum f) Cleaning with Garnet mesh 120 and surface finish with Glass beads

Probe a), c) and e), show a rough surface after cleaning with quite some residue from the additives. The

worst results regarding the residue are obtained using Corundum. This is mainly due to the angularity of

Al2O3.

The best results regarding surface quality and residue are achieved in test f), cleaning with Garnet

mesh 120 and finishing with glass beads.

In a second step, the same tests have been made using Suspension Technology cleaning with the additives

Corundum and Zirblast.

1 See Reference [1] – [4]

2 See Kachler-Report Nr. 80407001 and 80714001

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50x magnification 200x magnification

Dry blasting

Garnet 120 mesh

+ Glass beads

Suspension

technology

Corundum mesh

220

Suspension

technology

Zirblast 63µm

Figure 1 Scanning electron microscope: Garnet and Glass beads (dry blasting), Corundum and Zir-

blast (Suspension technology)

The surface roughness has been measured in three points on the surface of the cleaned bolts (corundum

and Zirblast) and on the surface of a new bolt for comparison.

The measurement results are:

Ra 1,42m – 1,48m with Zirblast

Ra 1,52m – 1,61m with corundum

For comparison, the surface roughness after conventional sandblasting is approximately Ra=3m.

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As can be seen in Figure 1 and Figure 3, the resulting surface roughness is mainly due to the initial manu-

facturing process (turning). The resulting surface roughness on a very smooth initial surface is shown in

chapter 2.2.2.

Figure 2 Cleaned stud bolts (above-corundum, below-Zirblast)

Figure 3 Surface roughness after cleaning with Suspension technology (left-corundum, right-

Zirblast)

Remark: The marks on the bolt surface are not caused by the cleaning process but due to the machining of

the bolt. The cleaning direction is perpendicular to these machining marks.

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2.2.2 Surface quality: Application of Suspension technology cleaning with different abrasives

Further tests have been accomplished to define the surface quality starting from a very smooth surface.

Starting with a surface roughness of Ra between 0.15m and 0.25m (grinded surface), different rough-

ness’s depending on the abrasive have been obtained as showed in the table below.

Garnet Mesh 220:

~Ra 1.8m

Corundum Mesh

220: ~Ra 1.0m

Zirblast 125 my:

~Ra 0.7m

Zirblast 63 my:

~Ra 0.3m

Surface quality after Suspension Technology cleaning

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2.3 Surface deformation

2.3.1 Lateral polished section

By analysing the cross section of the blasted specimen the deformation caused by the process is visible. The

deformation of the surfaces indicates possible luting of cracks or induced stresses.

Figure 4 Dry blasting (garnet) Figure 5 Suspension Technology (corundum)

Figure 6 Dry blasting (glass beads) Figure 7 Suspension Technology (Zirblast)

There is a clear difference in the deformation depth between dry blasting and using Suspension Technology.

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2.4 Blasting residues

2.4.1 Scanning electron microscopy

50x magnification 200x magnification

Dry blasting

Garnet 200

mesh

Dry blasting

Garnet 120

mesh

+ Glass

beads

Suspension

technology

Corundum

mesh 220

Suspension

technology

Zirblast

63µm

Figure 8 Residues on the surface

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By using a contrast analysis abrasive residues and remaining scale are made visible (see Figure 8). Using

the Suspension Technology process, the surface quality, degree of cleanliness and amount of residues on

the blade surface are improved compared to all the tests using the dry blasting method. The best results

have been achieved using Zirblast, where no residues are visible.

2.5 Luting of cracks

Luting of cracks caused by the cleaning process is a major issue. Therefore tests have been carried out to

analyze the luting provoked by Suspension Technology process and the dry blasting process as well for

comparison. The material of the test specimens is 1.4541 (austenitic steel).

2.5.1 Tests using dry blasting

The probes have been inspected by dye penetrant inspection according to specifications before and after

cleaning using dry blasting (with glass, garnet and glass and garnet). The probes before and after blasting

are shown in the picture below.

Figure 9 Probes before blasting (left), probes after blasting (right)

Using the conventional dye penetrant inspection method according to specifications not all cracks have

been detected even though they can be detected by visual inspection. Comparing the probes before and af-

ter cleaning, it is evident that cracks are missing on the probes after cleaning using the dry blasting method.

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2.5.2 Tests using Suspension Technology cleaning

The same tests have been carried out using Suspension technology cleaning with various abrasives: Zir-

blast, garnet and corundum.

Figure 10 Probes before cleaning

Figure 11 Probes after cleaning with Suspension Technology

Using the conventional dye penetrant inspection method according to specifications and depending on the

used abrasive not all cracks have been detected either using the Suspension Technology cleaning method.

The results with garnet and corundum are very satisfying as all significant cracks are visible.

Using the more time consuming inspection method with fluorescent dye penetrant, the cracks are visible as

even when using Zirblast (shown in the Figure 12).

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Figure 12 Probes after cleaning with water jet, fluorescent dye penetrant

3. References

3.1 International standard

[1] Non-destructive testing - Penetrant testing -Part 1: General principles (ISO 3452-1:2013)

[2] Non-destructive testing - Magnetic particle testing - Part 1: General principles (ISO 9934-1:2015)

[3] Non-destructive testing - Ultrasonic testing - General principles (ISO 16810:2012)

[4] Non-destructive testing - Eddy current testing - General principles (ISO 15549:2008)

3.2 External reports

[5] Zentrum für Wekstoffanalytik Lauf GmbH, Nassstrahlen von Dampfturbinenkomponenten

Werner G. Kachler, 2009-10-04,

file: 9100432 Nassstrahlen von DT-Teilen.pdf

[6] Zentrum für Wekstoffanalytik Lauf GmbH, Nassstrahlen von Dampfturbinenkomponenten

Werner G. Kachler, 2009-10-04,

file: 9100432 Nassstrahlen von DT-Teilen.pdf