How to correctly select precision stainless steel tube fittings?

Stainless steel precision tubes are generally used in precision instruments or medical devices, not only the price is relatively high, and usually used in critical equipment and instruments, so the material and precision requirements of precision stainless steel tubes and surface finish requirements are very high. Usually civilian precision stainless steel tube 301 stainless steel, 304 stainless steel, 316 stainless steel, 316L stainless steel, 310S stainless steel. Generally more than the production of NI8 above the material, that is, 304 above the material, not the production of low material precision stainless steel tube, customarily, will be 201, 202 called stainless steel, because of its magnetic, to the magnet has suction. 301 is also non-magnetic, but in the cold working with magnetic, to the magnet has suction. 304, 316 is non-magnetic, no suction to the magnet, does not stick to the magnet. The main reason for magnetic or not is that the stainless steel material containing chromium, nickel and other elements of the composition of different proportions and different metallurgical structure. Combined with the above characteristics, the use of magnets to determine the quality of stainless steel is not a viable method, but this method is not scientific, because the production process of stainless steel, cold drawing, hot drawing, post-treatment is better, then less magnetic or no, not good, then the magnetic larger, reflecting the purity of the stainless steel is not high or low. Users can also judge from the packaging of precision stainless steel tubes, appearance: roughness, thickness uniformity, whether the surface has a stain point point and so on.