The comparison of welded tube and seamless tube.

Due to the machining accuracy, the specification  of the tantalum tube will be influenced, especially about the outer diameter and inter diameter. There is also big difference of  the specification exited between the seamless  tube and welding tube, which will be directly affects the performance of tube.

Due to the machining accuracy, the specification  of the tantalum tube will be influenced, especially about the outer diameter and inter diameter. There is also big difference of  the specification exited between the seamless  tube and welding tube, which will be directly affects the performance of tube.
At present, the wildly specification of the tantalum tube used in the Petroleum and chemical equipment Is about  ∅19.05×0.381mm, including seamless tube and welding tube. The wall thickness will be getting more and more thin during the process of using in the corrosive working environment. When the place of the wall where is the thinnest of the tube  getting more and more thinnest to the critical thickness, the tube will break because of the different pressure between the outer and inter of the tube. For the seamless tube, the corrosion to the wall can be regarded as uniform, the failure position will be generally at the thinnest area wall. But regarding to the welded  tube, the speed of the chemical Corrosion and intergranular corrosion in the welding area will be  much faster than the area of No-welding, which will greatly influence the service life of welded pipe .
The requirement of tolerance on the specification according to the ASTM standard is as follow respectively: Seamless tube: ±10%, welded tube: ±5% (according to the plate standard). The bigger fluctuates on the wall thickness, the more serious of wasting will come into being. In accordance with the minimum requirements of pipe standard, Assuming the limit of tolerance happened in the same pipe, the wastage-rate will be different between the seamless tube and welded tube as: seamless tube 20%, welded tube   10%. By improving the production process and the testing of finished product, Our tolerance of seamless tube can be controlled in ±5%, Which greatly improved  the utilization rate of material, and reduce the waste rate to 10% same as the welded tube.
The Mechanical performance index of the welded tube has been weakened  to a certain extent because of the influence of the welding crack (the attenuation will be about more than 10%, and it will be much more o f the welded tube unannealed.) So the seamless tube in the mechanical property is better than the welded tube.
The chemical property of our seamless tube is as following: Δ0.2＞290MPa，Δm＞360MPa，A＞30 %, which is better than the ASTM STRAND: Δ0.2＞193MPa，Δm＞276MPa，A＞20%.
Hence we can know that, the critical thickness of the seamless tube that our company produce can be much less than the ASTM required, meanwhile the  workable thickness of the seamless tube will be more than the welded. With regard to the same specification of the seamless tube and welded tube, that means the utilization of the seamless tube is better than the welded tube .   
Regarding to the above analysis, we can fix out the element of the material  utilization  :
The minimum practical tolerance of wall thickness according to the ASTM standard: standard specification of seamless tube is 90% D (D is  wall  thickness in  theory ), but we can reach to 95% D, same as the 95%D of the welded tube.
The critical min wall thickness in the condition of security use: Standard seamless tube is d ,while the standard welded tube is 1.1 d,. (the welding process will reduce 10% of the mechanical property). So the actual wall thickness for corrosion preventive is: standard seamless tube - 90% (D-d) , seamless tube that we produce -95%(D-d) and standard welded tube -95% (D-1.1d).
The speed of the chemical Corrosion and intergranular corrosion: The standard seamless tube - υ, the standard welded tube - kυ, k is coefficient of corrosion of the welding area, k > 1 under normal circumstance .
The actual working life: The standard seamless tube -(90%(D-d）)/υ, the seamless tube that we produce -(95%(D-d)/υ, the Welded tube - （95%(D-1.1d)/kυ.
The price in the market: The seamless tube that we produce is about 580usd/kg, the welded tube is about 725usd/kg.

Please kindly check the following table:    

Item	Standard seamless tube	Standard welded tube	CSTN seamless tube
The min tolerance of  wall thickness	90%D	95%D	95%D
The critical thickness	d	1.1d	d
The actual use of the   thickness	90%D-d	95%D-1.1d	95%D-d
The rate of corrosion	υ	kυ（k＞1）	υ
The actual working life	（90%D-d）/υ	（95%D-1.1d）/kυ	（95%D-d）/υ
The price on market	580usd/kg	725usd/kg	580usd/kg
The cost performance	（90%D-d）/3600υ	（95%D-1.1d）/4500kυ	（95%D-d）/3600υ

 
Through the above analysis, we can get the conclusion that to compare the cost performance between seamless and welded tube, we have to  confirm the figure of the “d ”-the critical thickness and the “k”-the coefficient of corrosion of the welding area. But in the condition that the tolerance of the seamless tube can be controlled at the same level as the welded tube, the cost performance of the seamless tube is much more better than the welded tube.