How to Distinguish the Quality of Seamless Stainless Steel Pipes

Seamless stainless steel pipes are referred to as seamless pipes, with rough surface and indefinite length. Commonly used in boilers, central air conditioners, steel structures, mechanical processing, pipeline gas, fire fighting, etc. With the wide application of seamless stainless steel, there are more and more product types in the market. So, how to distinguish the quality of seamless stainless steel pipes?

Main distinguishing method:

(1) To identify imported stainless steel materials or ordered by steel mills, generally only need to check the marks on the steel or packaging according to the quality certificate of the import or steel mill.

The quality certificate is the supplier's confirmation and guarantee of the inspection results of the batch of products. Therefore, the quality certificate not only states the name, specification, number of pieces, weight, and delivery status of the material, but also all the inspection results of the specified guarantee items.

Similarly, in order to facilitate management, avoid confusion and prevent use accidents due to confusion, the manufacturer marks the brand, batch number, status, specifications, quantity, and manufacturer code on the materials or packaging. The marked mark should be consistent with the content of the quality certificate. The commonly used marking methods mainly include the following three types: painting (paint the color indicating the brand on the specified part of the material), printing (printing or spray printing on the specified part of the material to indicate the material's brand, specification, furnace number, etc. , Often used on thick steel plates or large and medium-sized steel), listing (on bundles or boxes of materials, hanging signs indicating the brand number, batch number, specification, quantity, etc.).

The signs of stainless steel are generally printed and listed.


(2) The society has overstocked stainless steel materials. According to the length of the overstocking time and the quality of storage, there are generally two situations:

One is that the overstocking time is not long and the storage is good. The processing unit not only keeps the original quality certificate or copy, but also the signs on the steel or packaging are complete and clear. For the identification of this kind of stainless steel, as long as the mark on the steel or packaging is also checked according to the quality certificate.

The other is that the backlog is long and the storage is not perfect. There is no quality certificate, and the marks on the steel or packaging are not obvious or fall off. For the identification of this type of stainless steel, it is mainly necessary to conduct in-depth investigation and research and consult the original documents with the processing unit. Generally speaking, the original documents such as invoices are marked with steel numbers. If there is no record of the steel number, the local price data at that time must be consulted to determine the steel number by price. This is a more effective way to determine the steel grade, but it is often easy to show up. Therefore, it is necessary to master the identification method of the physical object.

Physical identification is to determine whether it is stainless steel with the help of simple instruments and the senses according to the inherent physical and chemical properties of the product (including the whole material, residual material, waste, etc.) when the steel grade (number) is unknown. And the specific method of which type of stainless steel. 

It should be pointed out that sensory identification cannot distinguish the specific steel (type) number, but can only basically distinguish three categories of chromium stainless steel, chromium nickel stainless steel and chromium manganese nitrogen stainless steel. The identification methods are as follows:

1. Identification of color

The pickled stainless steel has a silvery white and smooth surface color: chromium-nickel stainless steel is silvery white and jade; chromium stainless steel is slightly grayish and weak; the color of chromium-manganese-nitrogen stainless steel is similar to that of chromium-nickel stainless steel. The surface color of stainless steel that has not been pickled: chrome-nickel steel is brown-white, chrome steel is brown-black, and chromium-manganese-nitrogen is black (the three colors refer to the more heavily oxidized colors). Cold-rolled unannealed chromium-nickel stainless steel with silver-white reflective surface.

2. Identify with magnetite

Magnets can basically distinguish two types of stainless steel. Because chromium stainless steel can be attracted by magnets in any state; chromium-nickel stainless steel is generally non-magnetic in the annealed state, and some will be magnetic after cold working. However, high manganese steel with higher manganese content is non-magnetic; the magnetic situation of chromium-nickel-nitrogen stainless steel is more complicated: some are non-magnetic, some are magnetic, and some are non-magnetic in the longitudinal plane and magnetic in the transverse plane. Therefore, although magnets can basically distinguish between chromium stainless steel and chromium-nickel stainless steel, they cannot correctly distinguish some special steel grades, let alone specific steel grades.

3. Identify with copper sulfate

Remove the oxide layer on the steel, put a drop of water, wipe it with copper sulfate, if it does not change color after wiping, it is generally stainless steel; if it turns purple, non-magnetic is high manganese steel, and magnetic is generally ordinary steel or low alloy steel.

For steel grades with special properties, we still need to take the following three methods for identification.

1. Identification of grinding flowers
Grinding identification is to grind stainless steel on a grinder and observe the sparks. If the spark is streamlined and has more dense knots, it is high manganese steel or manganese nitrogen steel with higher manganese content; if there are no knots, it is chromium steel or chromium-nickel stainless steel.

2. Identification by annealing method
Cold-worked chromium-nickel stainless steel, if it has magnetism, can take small pieces to boil it red in a fire and let it cool naturally or put it in water (annealing). Generally speaking, the magnetism will be significantly weakened or disappear completely after annealing. However, some chromium-nickel stainless steels, such as Cr18Ni11Si4AlTi steel and Cr21Ni5Ti steel, contain more ferrite elements, and a considerable part of their internal structure is ferrite. Therefore, it is magnetic even in the state of hot working.

3. Identification by chemical qualitative method
Chemical qualitative method is an identification method to identify whether nickel is contained in magnetic stainless steel. The method is to dissolve a small piece of stainless steel in aqua regia, dilute the acid with purified water, add ammonia to neutralize it, and then gently inject the nickel reagent. If there is a red velvet-like substance floating on the liquid surface, it means that the stainless steel contains nickel; if there is no red velvet-like substance, it means that there is no nickel in the stainless steel. The content is not easy to reveal or determine how much, generally it can be grasped after many experiments with standard samples).

The above identification methods show that the use of sensing tubes to identify stainless steel requires not only a comprehensive test of several methods, but also the test results can only determine a certain type of stainless steel, and can not determine which alloy elements and specific content the steel contains. Therefore, the method of sensory identification is currently extremely imperfect, and some may be wrong, and there are many physical phenomena that are only known but do not know why, and need to be further explored.

Therefore, if you want to correctly distinguish the best way to distinguish stainless steel grades, you need to make sufficient preparations, conduct in-depth research, and ask the relevant testing units to conduct testing and judgment. Only in this way can we guarantee the purchase of high-quality seamless stainless steel pipes.


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