Management System of Welding Process
in Rolling Stock Factories
By TONG Woon Chi
Introduction
A train set consists of car bodies connected together via couplers and linked with electrical cables. Each car body is made up of four major modules, namely, underframe, sideframe, roof and front/end frames. Both ends of the train set are the cab where the driver will take control of the train through on-board train control equipment. In terms of structural integrity, underframe is a critical module because it holds various mechanical and electrical equipment, such as auxiliary power equipment, traction control, compressor, etc. and is subject to vibration when running. Another major key system of the train set is the bogie where wheel and axle, gear box, brake and suspension device are mounted. It bears all the loads due to the car body and the passengers and vibration loads when the car is running along the track.
Design
Steel car body is made up of steel column, beam, channel, etc., which are welded together to form a rigid frame and then covered with steel sheet by welding to become the exterior surface. Aluminum car body is made up of aluminum extrusions which are welded together to form the whole body. Bogie is made up of steel plates and parts which are welded together to house various equipment to form a complete unit.
In view of the extensive application of welding in the fabrication process of car body and bogie, welding quality plays a critical and significant role in determining the service life of a train. As a train carries large number of commuters daily, its safety performance is of paramount importance. The European Committee for Standardization released the EN15085 series standards in 2007 to govern the design of a train in terms of its safety performance.
The EN15085-3 “Railway applications – Welding of railway vehicles and components – Part 3: Design requirements” specifies the basic principles for designing a train and its components.
The major considerations in welding processes are the stress levels the weld joints will experience and the safety categories, in case of failure, they belong to. With these two factors taken into account, a weld performance class is defined for each individual weld joint so that the type of NDT inspection to be executed is defined in order to ensure the weld quality meets the requirement.
Below is an extract of EN15085-3 showing the weld performance class and corresponding NDT inspection required.
Most designers will carry out FEA (finite element analysis) to determine the stress level experienced by weld joints and their safety categories.
Manufacturing
In EN15085-4 “Railway applications – Welding of railway vehicles and components – Part 4: Production requirements”, it specifies the documentation to be used in production, namely, welding planning documents (i.e. working plans, welding sequence plans, test planning documents and welding procedure specifications).
The welding procedure specification (WPS) must be qualified in accordance with relevant EN standards. Below is the extract of EN15085-4 showing the different qualification standards for WPS.
Besides the necessity of having WPQR (welding procedure qualification record) to support WPS, this EN15085-4 also specifies the type of control to be executed in weld preparation, welding execution and post weld inspection. But who is responsible for these control activities? For a welding management system to EN15085 series standards, there is a competent and qualified welding engineer appointed by the manufacturer as the “Welding Coordinator” who will be responsible for managing welding process, defining the activities to be monitored, appointing competent workforce to carry out welding and monitoring functions, determining the type of tests to be carried out when assessing welds suspected of known defects, and issuing instructions for repair, etc. The role and responsibilities of welding coordinator have been specified in EN15085-2 “Railway applications – Welding of railway vehicles and components – Part 2: Quality requirements and certification of welding manufacturer”.
When the manufacturer applies for certification to EN15085 standards and fulfils all relevant requirements, the name of the welding coordinator and type of welding process accredited in that organization will be listed in the certificate. This certificate is a proof that the manufacturer has operated a welding management system to achieve the desired weld quality and final product quality to boost buyers’ confidence.
The manufacturer is subject to surveillance examination by independent certification bodies to evaluate how well they are doing in managing the weld process to meet the EN15085 standards. This certificate will be renewed every three years
Quality and Inspection
In EN 15085-5 “Railway applications – Welding of railway vehicles and components – Part 5: Inspection, testing and documentation”, it specifies the qualification of inspectors for carrying out weld inspection. In particular, NDT inspection requires specific qualification levels, namely, Level 1, Level 2 and Level 3. NDT inspector of each level will have specific roles and responsibilities, while there are specific tasks they are allowed to execute.
Level 1 inspector is only allowed to carry out NDT inspection under guidance and not permitted to interpret the inspection results for drawing conclusion. Level 2 inspector can assess the inspection result against the acceptance criteria of the inspection instruction which has been prepared, approved and issued by Level 3 inspector or under a standard inspection procedure. Level 3 inspector is of the highest class and can draw a specific inspection instruction for a particular weld if the standard inspection procedure is not applicable.
Below is the summary of inspection level required in accordance with EN15085-5.
There is a particular requirement in this EN15085-5 standard. It requires the welding coordinator or his designate to carry out first article inspection at source where parts or components are produced by suppliers or sub-contractors.
Welding Management System
Under the EN15085 certification scheme, manufacturer is required to establish, implement and maintain a management system for controlling the welding process through organization setup, competence and qualification of personnel involved in the process, documentation, verification and validation of welding process and quality inspection.