The NISSAN foundry in Cantabria (Spain) has been manufacturing a complex ductile iron housing for JOHN DEERE for several years. This casting is a critical component of the hydrostatic drive trains used to power crawler tracks used in civil engineering machinery.
The T 239237 housing is part of a transmission system assembled by JOHN DEERE in their Getafe plant, close to Madrid. This is a core product of the plant, and is one of 5 critical castings that NISSAN Cantabria produces for this project.
The casting (T 239237) also known as “the tear”, due to its shape, is infamous within the foundry, primarily because of previous problems related to porosity. Even with an “approval concession”, many castings were still rejected due to porosity revealed during machining, reducing productivity and efficiency for both the foundry and the customer.
Over the years many attempts were made to address the porosity issues, with changes in the gating and feeding systems. These initial attempts were generally unsuccessful and for the foundry, the casting began to live up to it’s name – “the tear”.
In basic terms, the problems with the casting were due to geometry, relatively high weight and the material specification (GJS 500-7).
The casting geometry (Figures 1 & 2) highlights significant changes in the section thickness, generating two different cooling and solidification volumes. This leaves the inner ring completely isolated and difficult to feed, resulting in gross porosity defects.
The porosity defects were always located in the central part of the casting, as shown in Figures 3 & 4.
The foundry conducted trials with several process parameters to eliminate the problem:
- chemical analysis
- pouring temperature
- adjusting the metallurgy
- feeder system modification
None of the above completely resolved the problem, so the foundry decided to use a chill on the inner profile of the casting, to move the defect to a more acceptable location.
Different methoding designs were studied with the objective of placing chills in the inner and outer parts of the casting, to investigate the effect on cooling. (Figures 5 & 6)
Even with the use of chills, the required quality standard could not be consistently achieved, with scrap levels after the final machining of nearly 2.5 % – this was still unacceptable, due to the high cost of machined scrap.
Another problem associated with the use of chills was how to remove the inner chill after casting. Initially, the foundry had to use a manual system to remove the chill, see Figure 7. This practice was costly and difficult, and unsatisfactory from the safety perspective. Subsequently, a hydraulic press (Figure 8) was used; this improved the safety aspect but created a different set of problems with the risk of marking the casting surface to a depth beneath the machining allowance. Even applying a coating on the chills, small defects occurred which gave defects in the machined areas of the casting.
The production of the chills was also a problem for the foundry, these had to be cast in-house and in many cases it was not possible to re-use them.
In 2009 JOHN DEERE proposed a change in the design of the casting. It was a small change in the geometry of the casting, but the time required for the re-approval process, presented Foseco with an opportunity to develop a new methoding proposal for NISSAN.
A joint project was established between NISSAN Cantabria and Foseco Española, with the aim of solving the existing shrinkage problems and improving the quality and consistency of the casting.
The work started with a solidification and mould filling study using MAGMASOFT®. There were two clear objectives:
- eliminate the use of chills
- evaluate the use of highly exothermic FEEDEX spot feeders (which could now be applied due to the casting design modifications)
The chosen option was to feed the external profile of the casting with a KALMINEX* 2000ZP 9/12 K sleeve; whilst using a FEEDEX VSK 770 / 33 MH sleeve (modulus 4.2cm) to feed the internal part of the casting. This combination of feeders gave the foundry several advantages – high yield, improved moulding properties and an extremely small contact area with the casting (leading to a subsequent reduction in fettling time).
In addition to the improved feeding system design, the cores were manufactured with 30% of Chromite sand in the core sand mix to improve the temperature gradient during the solidification process. As a result, the central core acted as a weak chill, reducing the superheating effect in the central part of the casting. (Figures 9 & 10)
Based upon the MAGMASOFT simulation predictions, it was decided to produce a series of test castings to confirm whether the shrinkage problems had been resolved.
The results of the tests were, from the beginning, very promising and the castings were completely sound and clear of internal defects (Figure 11).
The results of this project have been very satisfactory for both the NISSAN foundry and their customer JOHN DEERE. For NISSAN an important and expensive scrap problem has been resolved, as the defect was not detected until after the final machining process. Importantly, the problem has been addressed without an increase in the overall casting cost. The use of FEEDEX K spot feeding technology has significantly improved casting yield, reduced fettling costs and eliminated the need for conventional chills. With the scrap percentage now running at a very low level, this project illustrates the effectiveness of the partnership between NISSAN Cantabria and Foseco Española.
Foseco would like to express sincere thanks to Javier Conde (foundry engineer) NISSAN, Cantabria, for his help and support in authoring this paper.