The molds / cores have been assembled and are now ready awaiting the pouring of the molten metal. Preparation of the ladles can be seen with attention to the pouring lips. Some are short and stubby whilst others declare a more gentle lip is best to pour. The end user should be raising questions on the thermocouple with visible evidence that a new tip is being used. Do check the instruments calibration is valid. Depending on the geographical location of the foundry there will be a diversity of personal protection equipment (PPE) from very good to very poor. Operators with scars can often be seen. Whatever the situation make sure you as a visitor have been given the correct personnel protection and stand a suitable distance from the pouring.
The raw material melts and the ladle charge commences. The importance of “good” raw materials cannot be over emphasized.
Regular samples are taken from the ladle for spectrometer analysis. Ensure the test laboratory is set up correctly with the latest ASTM Specifications and equipment is certified etc. The number of times I have seen hand written sheets for controlling the properties is amazing. Remember all aspects of the foundry should be using certified /controlled standards. So the analysis is done and chemical components are added. Normally there is a storage area holding the chemical components. The weights have been estimated and a certified scale should be in the vicinity to ensure the correct additions are added. Yes you’ve guessed…the weighing is missed and “experience” comes into play. The slag / impurities on the molten metal top are continually skimmed away. The molten metal temperature is checked to be within allowable tolerance. Everything is ready to pour… can’t you just feel the excitement!). Do check the measured temperature by the operator against that stated on the control charts / specification. A common issue is the temperature change in the pouring. Also aspects of de-oxygenation come into play.
The molten metal is either poured directly or via a smaller mobile ladle into the molds. Timing and pouring rates are critical to maintain acceptable molten metal temperature. This can be quite spectacular with white, yellow, orange molten metal splattering and smoke arising from the mold risers. The heat can be felt. Sometimes with investment castings the molds fail and the metal pours onto the foundry floor. Failures of sand castings are rare except when you break open the mold to reveal the metal flow has not been correct to all aspects of the mold. The end user should note such failures to promote discussions and understand the foundry’s response.
At this time test bars are also poured which are representative of the cast valve component. They have a unique numbering so each casting can be uniquely identified and traced. Test bars should be poured with every heat and ladle. Sometimes foundries try to reduce the number of test bars when repeat pouring is done during a shift or if the ladle size is small, say for the high end materials. Such a practice is not acceptable to many end users and, more recently, to the latest casting standards such as MSSP147.
The test bars have to go through the same process as the casting with regards to heat treatment. The location of the test bars in the furnace should be discussed with the foundry. The “best location to get the best results” is placing the test bars at the top of the furnace charge but this may not be considered as truly representative of the whole charge. Whatever their location it is important to ensure it is representative of the actual castings being heat treated such that mechanical and chemical properties from the test bar are correct. Regretfully there have been many instances in operation plants where this has been found not to be the case. Mechanical, chemical, low temperature properties have been found to be in error which has resulted in the end user having to remove 100’s and 1000’s of valves from service. Some end users request extra test bars which are kept for future reference and testing. In some foundries the above operation is at night time. Usually this is to use cheap energy.
I must admit I became frustrated having traveled for days to witness the pouring to be advised it had already been done. I see empty ladles. So now I send advance requests to the foundry and if need be I skip the dinner to witness the pouring, chemical analysis etc.
Pilot castings & production castings
So far the above has described the general process for pouring without any differential between “Pilot” and “Production” castings. Currently within the foundries such requirements do vary considerably on when to, or when not to, pour “Pilot” castings. Often existing “Pilot results” are “extended” to cover other castings. Even to the extent whether to allow weld repair or not.
I refer you to MS-SP147 where excellent criteria are given. I will discuss this further later. So far some of my discussions on MS-SP147 with the foundries have been most interesting even for those that are considered acceptable. The general dislike is the standardization aspect of pilot testing and production testing with the demonstration of correct manufacturer controls and testing. The end user auditor should familiarize themselves with MS-SP-147 as it’s an excellent aide memoir when discussing and understanding the foundry’s practice. Benchmarking is of value to establish any gaps in the foundry practice. The end user community is challenged to include MS-SP147 in their purchasing documentation. Where this is implemented casting quality will improve and risk to plant operations significantly reduced.
Removal of risers & downcomers
The next stage is the removal of the sand/ mold casing, cutting/ grinding off risers and downcomers. Yes it can be very noisy. During this activity you can begin to see the quality of the casting from what is seen on the surface. It is really encouraging to see good surfaces as it is likely that the casting itself is good internally. As a rule of thumb the more surface grinding and weld repairs seen, the differentiation between a good casting, an acceptable casting and poor castings can be easily established. Whatever is seen at this stage should confirm any foundry statements on their control of quality.
Once again I appear to have let my “pen” run away with me. It’s amazing how much there is to share on castings which, by the way, is really just an introduction.
Thank you for your continued support. Please contact me if you have any questions or different views. They are most welcome.
Barrie Kirkman, BSc CEng MIMechE, writes a regular column for Valve World, bringing his own personal views from inside the valve business.
Barrie can be reached on: email@example.com.