Depending on the foundry casting design can be based upon “experience” and /or “Computer Aided Design” that simulates the pouring and cooling down process. Here risers and downcomers are evaluated to optimise the required casting parameters. The end user should challenge this area to see how the foundry uses such software to resolve casting issues by examples. It is important to relate the analysis with the actual pouring.
When visiting the poured castings area the end user should note any mould failures or amount of weld repairs as this would be a good opportunity to challenge the foundry’s technical competence. Do they really collect quality data, analyse it and use computer aided design to improve? Or is it by “experience?.............
The “experience” design should be taken with caution. Indeed in many cases this is acceptable but methods to succeed with improvement are more along the lines of trial and error. Often optimisation of the design is wanting.
The knowledge of “thick wall” and “thin wall” castings should be known together with the required pressure wall thickness and incorporated safety factors. Care should be taken to ensure smooth transitions at discontinuities. It is required to understand the corrosion allowance and potential impact of going beyond 3mm. Some end users state on their piping specifications 6mm rather than the selection of the next grade of material. There are also important differences between carbon steel, low alloy, stainless steel and exotic materials. Be aware of translating carbon steel designs straight to low alloys or similar without the appropriate checks. Believe me it’s often done.
These vary depending on the casting process such as lost wax or sand castings. Wooden patterns are the most common for sand castings, while metallic patterns are used for lost wax castings with wax injection, shell making and then dewaxing. What ever the process the foundry should implement an effective manufacturer and storage with correct indexation. Too often the pattern warehouse is a pile of patterns loosely controlled. Inspection of the patterns informs the end user of the various brands being poured. This is good intelligence. This is also where you confirm whether badgeing is practiced.
That is where the main wooden patterns have a recess to insert brand names for example “Bazza”. This means the design of castings are the same for all brands other than the brand name. I have seen foundries displaying 20 to 100 such badges. Often the brand names are familiar but sometimes there are a few surprises! Badged castings are easy to pick out as often there is a raised rectangular line where the molten metal has flowed into the gap between the badge and main body. Some end users will only approve non-badged valves.
Generally the move from carbon steel to stainless steel, low alloy and duplex requires greater control on the raw materials and their sources. What is the recycle percentage of poured materials? Having just said what I have just said is being aware of the “cube compressed raw materials” as you have no knowledge of its content. There needs to be an acceptable level for carbon steel where certification should be supplied then the other materials likewise follow. Even here the foundry should operate a quality system that indicates which raw materials have been accepted and those awaiting inspection. The raw material storage area should have identified segregated storage areas with no risk of cross contamination. When examining this area ensure it is clean with no “rogue” materials lying outside the storage areas. I have seen the materials stacked so high such that have fallen into the next bay. A colour coding system is operated by the better foundries to maintain traceability from storage to the preparing of the load charge. Yes even carbon steel, LCC and LCB require such a coding system. As you move towards the higher end materials the load charges tend to be smaller and the need for good control even more important.
For investment castings metallic patterns are used for lost wax castings with wax injection, shell making, and dewaxing. Beware if it’s the first time you are visiting this type of foundry. The release of ammonia can be high often causing eye and breathing irritation. The more modern plants have reduced such emission considerably. The question of health and safety is raised for the workers with no protection to simple paper masks and then the more sophisticated breathing mask. The methods of drying the moulds and appropriate temperature and humidity control are fundamental for producing good castings. Some use simple fans whilst others have dedicated drying rooms with the mould circulating on a conveyor though a controlled temperature. The auditor can easily determine those moulds that have good control and those who do not. There are various types of sands for castings which have been selected to have specific properties with different casting quality. Some deliver an excellent surface finish to the MSS-SP-55 whilst others much less. Cost sometimes affects the selection. Always look at the sand mixing area to determine the mix of new and re-cycled sand.
The more experienced foundries test the compressibility of the sand to assure its effectiveness when loaded, say, from the molten metal. The test sample shape is symmetrical which may be cylindrical, cubical or rectangular. It’s worthwhile to request some latest test results as it has been observed that a foundry has the test machine but it is never used. Check the test machine’s calibration. Methods of pouring the sand into the mould holding the pattern can be literally by shoveling and then manually compressing by a large hammer process (or even with feet compression). Needless to say the quality of such sand mouldings is questionable as uneven compressibility of the sand is most likely. When the pattern is removed “hair line cracks in the mould” can sometimes be seen, which are locally repaired.
The more acceptable method is semi/ automatically pouring the sand into the mould with a machine hammering vertically from a single metal plate.
Where these practices are seen more confidence is gained in the casting quality than the “feet” option. In some instances the sand is hardened by gas injection. Here the skills of the foundry really do come into play as to me this is “special” to achieve a good casting. The number of, location of and gas injection duration all come into play. Some foundries ban the use of cameras and often keep shop tours away from this part of the process.There is also some secrecy on the coating / treatment of the mould surfaces and protection of the downcomer openings.
Once again I appear to have let my “pen” run away with me. In the articles, test bars, achieving correct chemical properties, surface finish, pilot castings, production testing, ANSI B16.34, NDT, weld repair, heat treatment and documentation will be discussed.
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.