About

Refurbishment of torpedo ladle and locomotion axles through Laser Applied Surface Engineering

Scope

The project aims to deliver a process that offers a coating which benefits from increased wear and fatigue resistance, coupled to an inspection method that ensures and validates the safety of the part prior to service. Such a coating would allow for refurbished axles to be in service for longer than the original un-coated components, thus reducing scrappage rates by more than a half. Since the axles weigh around 600kg, assuming the mean CO2 burden of steel is 1.77kgCO2/kg, the environmental cost of scrapping such a component stands at 1062kg CO2.

The loading mechanism on the axles are numerous, ranging from bending loads during discharge of molten iron from the blast furnace into the torpedo ladle, torsional loading during transportation, impact from inconsistencies in the rail network and numerous others. With this in mind, the testing programme needs to be designed to account for each eventuality in order to quantitatively assess the mechanical performance of the laser applied coating. Mechanical and non-destructive testing will need to be carried out for a number of powder and process developments in order to develop a final coated product that resists the phenomena of persistent slip bands arising due to cyclic slip as a result of cyclic loading.

A robust process for evaluating coating adhesion, the presence of porosity and other potential defects will be developed using ultrasonic techniques. These will include Phased Array (PA), Full Matrix Capture (FMC), Virtual Source Aperture (VSA) and submerged UT, where all technologies will be assessed and validated against X-ray computed tomography. High frequency surface inspection will be considered, as well as lower frequency through transmission. Other NDE techniques such as Eddy Current and EMATs will be considered. Once the NDT procedure is finalised, pre-use inspection of the part will ensure that the component is fit for purpose and free from any defects that may arise from any fluctuation in processing parameters.

Coating Technology & Application

Laser cladding involves welding metallic powders via a high powered focused laser onto a work-piece in a controlled manner, such that the correct levels of thickness, dilution and mechanical properties are achieved during the process. An image and schematic of the process are shown in figures 1&2, where an image of a refurbished axle is shown in figure 3 (post finish machining) shows an example of a laser applied coating applied to a worn axle. Development of this technology thus far has resulted in excellent coating adhesion and mechanical properties, as shown by the micrograph and micro-hardness results of a stainless steel coating in figures 4 and 5 respectively.

[ Zoom ]
Figure 1. Laser cladding process
Figure 1. Laser cladding process
[ Zoom ]
Figure 2. Laser cladding schematic.
Figure 2. Laser cladding schematic
[ Zoom ]
Figure 3. Laser applied coating (pre finish machining)
Figure 3. Laser applied coating (pre finish machining)
[ Zoom ]
Figure 4. Laser applied coating micrograph
Figure 4. Laser applied coating micrograph
[ Zoom ]
Figure 5. Micro hardness results
Figure 5. Micro hardness results

Validation procedure

TWI’s role in the project is to impartially demonstrate the safety of the finished product through destructive and non-destructive means. Assessment and continuous feedback of various powders and process parameters throughout the project will allow Tata Steel, LASE Ltd and Wall Colmonoy to develop a refurbished product which is fit for purpose and ready for commercialisation.

Destructive tests covers fatigue, wear and corrosion, where if possible the NDT procedure developed will be aimed at using off the shelf instrumentation, such as Olympus’ Omniscan (pictured) for ultrasonic/phased array/eddy current inspection. If this is possible, then it will provide a cost effective inspection solution for the coating partners to commercialise the process. If a more advanced option is required, then there are 2 options:

  1. Use of custom designed & manufactured probes. This option would not be much more expensive compared to using standard probes, as the main instrumentation (i.e. Omniscan) could still be used, but with higher frequency/smaller pitch probes etc. being used compared to what is commercially available.
  2. Use of more advanced signal processing techniques & post-processing. TWI have developed several advanced ultrasonic techniques which it could provide to partners within the project, where it is not envisaged that the cost of such an inspection procedure would be prohibitive to the commercial success of the coating technology.