Wed, 18 September, 2024
TWI was recently contacted by a representative from the Orwell Astronomy Society (Ipswich) (OASI) to look into the possibility of repairing part of the Tomline Refractor.
This telescope, which is around 150 years old, employs paired leaf springs made from an unknown steel. These paired tensioning springs are mounted within a recess on the outside edge of the objective lens cell assembly that houses the telescope’s 258mm diameter objective lens. They are located near a small rectangular opening, allowing access inside the cell. Their function is imparting a sideways force via the opening to a floating plate against the edge of the objective lens. This pushes the objective against two fixed supports inside the cell. All three supports are spaced at 120° intervals.
Figure 1A. The Tomline Refractor
Figure 1B. Orwell Park Observatory
However, as one of the springs broke during a recent manipulation, OASI asked TWI whether a weld repair may be possible. The broken spring had fractured at its centre, in a manner that, with a brief examination, appeared to be a result of fatigue. This is characterised by relatively flat and smooth fracture faces, with a different failure mode (typically brittle or ductile fracture) at the remaining ligament of the face, when the supported stress overpowers the small final area of material.
Figure 2. Broken leaf spring
Figure 3. Fracture faces of leaf spring
Given the age of the component, and lack of affiliated drawings, the composition of the steel was unknown. Although it is possible to determine the composition and likely thermal history of a piece of steel by suitable testing, such as optical emission spectroscopy, microstructural examination or replication, this involves both cost and destruction/damage of the part to allow for examination. In this scenario, a temporary fix using a relatively conservative approach was considered a suitable compromise.
With the assumption that the component was a relatively simple carbon steel, and the low volume of weld metal required, a manual tungsten inert gas (TIG) repair with an ERNiCrMo-3 (alloy 625) filler rod was performed.
ERNiCrMo-3 is a fully austenitic material, which is often used for welding unknown or dissimilar steels due to its ability to support a degree of contamination and also hold hydrogen in solid solution at higher amounts than ferritic materials. This reduces the risk of fabrication hydrogen cracking, which is the primary fabrication welding failure mode in carbon steels. It also provides a good colour match with most steels when polished, which can be important in terms of cosmetic applications.
Figure 4A. Repaired leaf spring
Figure 4B. Repaired leaf spring
The welded part did not show fabrication hydrogen cracking post-weld, and was successfully ground back to its original shape, allowing it to successfully re-used as a spring component.
The OASI stated that they were thankful for the sympathetic and well-engineered repair, and we were delighted to be able to assist with repairing a piece of astronomical history.