MIL

LEN

NIU

M S

TEEL

201

4

162

Non-destructive testing of bars by inductive heat-flux thermography Steel bars for extreme service or safety critical application require excellent surface quality. The traditional magnetic particle defect detection system suffers from a number of speed, detection capability and data processing drawbacks. Foerster has developed a significantly improved system, DEFECTOVISION® IR, based on thermography, which is faster and provides reproducible data on position and depth of defects.

The quality requirements of round and square steel bars for crankshafts, engine and chassis components

are extremely high due to their in-service and safety requirements. This means the bars are extensively tested for both internal and external defects.

For surface inspection a surprisingly high number of companies still work with manual magnetic particle inspection systems, which suffer from a number of drawbacks:

` The results are subjective` They lack good reproducibility ` Are very complicated to document` Are highly sensitive, which is why there are many

signals that do not supply information on the depth of the defect

` Suppression of defects below a certain threshold is not possible

` The method is slow.

Author: Stefan KochInstitut Dr. Foerster GmbH & Co. KG

r Fig 1 Schematic of system

Institut Dr. Foerster GmbH & Co. KG, an international, technological pioneer in the development and production of instruments for non-destructive testing of metallic materials, for the determination of material properties, or for the detection of metals, has developed an alternative approach using heat-flux thermography and called DEFECTOVISION® IR. This provides reproducible data on position and depth of defects, which is stored in a database and can be printed in a complete test report.

In the past, inductive thermography for the testing of steel billets captured the temperature at each surface point only once (a snapshot). The newly developed and patented evaluation principle is based on the dynamic recording of the temperature development – meaning multiple measurements of the same surface point are made within a given time interval. This, and the application of specific software algorithms, allows for a maximum certainty in the distinction between actual defects in the material and pseudo-signals. Bar dimensions from 50mm to 220mm can be tested.

The principle is shown in Figure 1. By applying the method of inductive heat-flux thermography and on the basis of patented software algorithms, an accurate, reproducible defect detection with an effective, simultaneous suppression of pseudo-signals can be realised at test speeds as high as 1.5m/s and speed is also unrelated to product size. This demonstrates the superiority of the new test system. There can be some limitations from roller conveyors, but not from the DEFECTOVISION IR itself.

TEST PROCEDUREBefore testing, the test piece is finely wetted with water for homogenisation and to increase the emission coefficient. An induction coil then heats the test piece by ~10°C via the induced eddy currents, and a further increase at surface-open defects, which is proportional to the depth of the defect. Four infrared cameras conduct a

FINISHING PROCESSES

MIL

LEN

NIU

M S

TEEL

201

4

163

gap- and contactless scan of the surface of the test piece (see Figure 2). Collected data is stored in real time. An example is shown in Figure 3. Bars are paint sprayed to highlight defects for subsequent rectification.

APPLICATION EXAMPLEThe Lech-Stahlwerke GmbH plant in Meitingen processes roughly 1.1Mt of steel per year and approximately 80% of the engineering steel and special steel produced is for the European automotive industry. This means the highest standards must be met and extensive testing needed.

The former system had encompassed several components of various suppliers, and problems arose at the communication interfaces when a defect was detected and then the line was stopped. Since the commissioning of Foerster equipment, the situation has changed because the complete system, including hardware and software, comes from one source.

For Lech-Stahlwerke, the system was customised to meet particular requirements. Particularly important factors for the customer were the reproducible detection of defects starting at a depth of 0.3mm and a length of 12.5mm, minimisation of pseudo-signals, automatic detection and marking of defects, as well as high test speed.

Figure 4 shows a DEFECTOVISION® IR system at Institut Dr. Foerster in Reutlingen, Germany.

The DEFECTOVISION® IR system is also particularly user-friendly, with interfaces to a level-2 system and the possibility of result storage in a database. Figure 5 shows a screen shot during a bar lot testing with the thermal images of all four cameras in the right corner and the positioning of the defects in the lower part of the screen.

Cost benefits The new test system has delivered improvements and an efficiency increase in production. The system is now permanently available, the test speed has been increased significantly, the defect dimensional detail is greater and operating costs are lower. The database material, generated during the test, can be evaluated in various ways. Furthermore, ongoing support of the Foerster specialists is available. MS

Stefan Koch is Product Specialist Thermography at Institut Dr. Foerster GmbH & Co. KG, Reutlingen, Germany

CONTACT: vogt.martina@foerstergroup.de r Fig 5 Screen shot example

r Fig 3 DEFECTOVISION IR sensor system: the ‘X-Box’

q Fig 2 Infrared image

r Fig 4 DEFECTOVISION® IR at Institut Dr. Foerster GmbH in Reutlingen, Germany