Stainless steel castings are widely used for industries such as food machinery, pumps and valves, pipe fittings, automotive, etc. Although current stainless steel investment casting technology is rather mature, it is avoidless to occur defects in the whole production process. In order to guarantee each cast stainless steel part is good, quality control is critical before delivery.
CFS Foundry offers a wide variety of quality verification and produc testing services to provide the highest quality cast products for our customers. So, how do we do quality control for stainless steel castings?
Stainless steel parts are formed by pouring melted stainless steel into molds. This makes it possible to cast products with different stainless steel grades. And each stainless steel grade consists of different chemical compositions with elements in a certain content, such as C, Si, Mn, P, S, Cr, Ni, Mo, etc. The error of any chemical composition may affect the usage of production. So, stainless steel casting manufacturer should test the material strictly. The quality control on material in CFS is: Material certificate is requested and provided when ordering raw material from our material factory. With in house spectrograph, our quality tester will test material carefully and the recorded result will prove if each chemical composition is within its range. And after casting, our testing instrument will confirm again if there is any change on the material compositions, and the testing result will be recorded in our laboratory room for future trace, also the material report will be sent to our customer.
Visual Inspection is the processs to find out the stainless steel castings with surface defects by visual. Following the industry standards of stainless steel castings and specific requirements from our customers, visual inspection should be 100% carried out, and manufacturers need to make sure there will be no defects on the surface of stainless steel castings. Common surface defects of stainless steel castings are: mile scales, burrs, air holes, sand holes, cracks, shrinkage cavities, etc. In CFS Foundry, any cast stainless steel part with such defects must be picked out and can not be shipped to our customers. And all the inspection result must be documented and stored to record all the products made in CFS.
Stainless steel investment casting is a custom process for parts in different shapes. According to the applications of stainless steel castings, there are different dimensions and tolerances requirements. Stainless steel process can help you to obtain precision products with ideal sizes and dimensions for its high accuracy. Sometimes, due to the limitations of casting process, post machining has to be done to reach higher surface finish or closer tolerances. For those critical dimensions, they must be inspected exactly. Below is the common dimensional inspection methods in our foundry:
1) Measuring tools–Measuring the dimensions and tolerances by tools is the most direct and simple way. To ensure qualified products, we need to test each important dimensions by measuring tools according to the drawing. Common used measuring tools for stainless steel castings are: vernier caliper, depth indicator, dial indicator, and so on.
2) Testing guage–Is made according to the sizes which will probably affect the assembly or use of products. For threaded holes, we can buy go and not go gauge according to the thread specification directly from the market. Testing guage is mostly used when the dimensions can not be tested by measuring tools.
3) CMM–Is the most accurate instrument to test the dimensions of products. But CMM is very expensive and only few foundries have such instrument.
Mechanical Properties Testing
When there is a higher requirement on the mechanical properties of stainless steel castings, heat treatment will be done. To confirm the properties of cast stainless steel part, mechanical properties testing is performed to approve the quality of products. The testing confirms the properties comply per the specified mechanical requirements.
Hardness testing–Is tested when there is a hardness requirement on stainless steel castings. Hardness testing result is mostly shown in HB or HRC.
Tensile testing–A destructive test process that provides ultimate tensile strength, yield strength, elongation and reduction area rate of the products.
Non Destructive Testing
NDT (Non Destructive Testing) allows us the capability of testing the quality of a stainless steel casting part without any destroying. Below are the common testing methods:
Dye Penetrant Inspection: Sometimes it is difficult to find the surface defects by naked eye, such as tiny cracks, pores or other surface defects. DPI is a testing method used for surface defects of all stainless steel castings. In this inspection process, an dye solution with color is applied to the surface of the castings. The dye, which is suspended in penetrating oil, finds its way into the surface defects. When a special developer is applied, the defects are clearly indicated under a special “blue” light. The solution is to detect any cracks or other surface defects due to porosity or gas that produced on the surface.
Magnetic Particle Inspection: It is a non-destructive testing method to find the defects by the accumulation of magnetic powder where the defect is located. MPI testing is convenient and affordable. It detects small cracks on or near the surface of investment castings. Cracks and defects have differing magnetic properties than those of the surrounding material. Their presence will interrupt the magnetic field, causing distortion, which indicates the shape and position of the crack or void.
X-ray Inspection: This non-destructive method is carried out to detect internal defects of cast stainless steel parts, like shrinkage and inclusions.
Ultrasonic Testing: It can evaluate and diagnose various defects like crack, inclusion and pinhole, etc. When the ultrasonic wave propagates in a job, one can detect the defect in it by the influence on the propagation of ultrasonic wave based on the acoustic characteristic demonstrated by the defect in the material.