Requirements for nuclear power valve castings

1.Requirements for casting material

The composition and mechanical properties of the real object shall meet the standards specified in the design and the selected material brand.

2.Dimensional tolerance, weight tolerance and surface roughness

Dimensional tolerance, quality tolerance and surface roughness are all important indexes reflecting the quality of castings, but the requirements for all kinds of castings are different.

(1) Dimensional tolerance of castings

It shall conform to the Chinese standard GB / t6414 dimensional tolerance and machining allowance of castings, which is divided into 16 grades. The tolerance grade CT (dimensional tolerance grade of castings) of castings shall be determined according to the material of castings and the selected production method, and the specific tolerance value shall be found according to the different nominal dimensions. Generally, 14 grades of clay sand and 11-12 grades of self hardening sand are selected.

(2) Weight tolerance of casting MT

The weight tolerance shall be in accordance with GB / t11351 tolerance on weight of castings, and 16 grades are also specified. The tolerance grade CT shall be determined as above, and the tolerance value shall be found according to different weights. General confirmation methods are as follows:

1) In mass production, 10 pieces shall be randomly selected by both the supplier and the demander, and their average value shall be taken;

2) The actual weight of qualified castings for small batch and single piece shall be confirmed by both the supplier and the demander;

3) Take the actual weight of the standard sample as the nominal weight.

(3) Surface roughness of casting M

Up to now, there is no grading standard for casting roughness in our country and no relevant standard in the world. At present, the parameters for evaluating surface roughness in most countries are the arithmetic mean deviation (RA) of the surface profile of the casting. The unit for measuring the roughness in microns (UM) requires special instruments. At present, all industrial countries use standard sample blocks for comparative evaluation.

3.Quality of castings

(1) Appearance quality of casting

Generally, the designer of the casting shall stipulate according to the service conditions of the casting, or the supplier and the Demander shall negotiate to determine the allowable defects on the casting. Generally, the defects shall be inspected by the inspector through visual inspection. Generally according to MSS SP-55 quality system for steel castings of valves, flanges and fittings and other piping components – visual inspection method for irregular surfaces.

If the surface quality requirements are high, it can be proposed that castings shall pass the penetrant inspection or magnetic particle inspection, and the two non-destructive tests shall be accepted as per Appendix B of ASTM B16.34.

(2) Internal quality of castings

Generally, it is not required to detect the internal defects of castings, so special products are required. There are two ways to inspect the internal quality of castings: one is the anatomy of castings, the other is non-destructive testing. NDT mainly includes Pt, MT, UT and RT.