Production system of weldable and stainless tubular structures with high mechanical strength and product obtained therefrom

Abstract

A production system weldable and stainless tubular structures with high mechanical strength and related obtained product, particularly for making tubular elements in cold-drawn stainless steel, workable at different thicknesses and shapes, provided with high performances in terms of mechanical characteristics and weldability, for the construction of light and ultralight structural frames destined for a dynamic use. It comprises a creation step of a tip for making the end of the tube smaller, so that it can pass through the drawing equipment and be coupled for drawing, a step of annealing heat treatment for softening the material and making it deformable. The system provides a mechanical test for evaluating the mechanical characteristics of the material and a metallography for viewing the structure of the material and evaluating if it comes within pre-established parameters for the drawing and a chemical preparation of the surfaces, for lubricating the contact surfaces of the tube with the drawing equipment and for avoiding seizures during the drawing. The preceding steps are repeated until the desired thickness. A step of final heat treatment follows, for reforming the structure of the steel and for fixing the desired final characteristics. Finally, a step of straightening is carried out for make the drawn and furnace-treated tube rectilinear, and a step of passivation for inducing a compact oxide patina ensuring its resistance to corrosion. The cutting, control and packaging operations follow.

1 . Production system of weldable and stainless tubular structures with high mechanical strength, comprising the following steps in succession:
 annealing heat treatment of a preform in martensitic or austenitic-martensitic steel,   chemical preparation of the surfaces for lubricating the contact surfaces of the tube with the drawing equipment,   at least one drawing passage for deforming the material in a permanent manner.   
   
   
       2 . Production system of tubular structures according to  claim 1 , comprising a step of final heat treatment for reforming the structure of the steel which was deformed and for determining the desired final characteristics. 
   
   
       3 . Production system of tubular structures according to  claim 1 , comprising at least two drawing passages until the pre-established thickness is reached. 
   
   
       4 . Production system of tubular structures according to  claim 1 , wherein in every drawing passage the thickness is reduced by about 20%. 
   
   
       5 . Production system of tubular structures according to  claim 1 , comprising, before said at least one drawing passage, a step of normalisation heat treatment of said tubular structure. 
   
   
       6 . Production system of tubular structures according to  claim 5 , wherein said step of normalisation heat treatment is conducted at a temperature in the range of 950° C.-1150° C. and for a time greater than 10 minutes and preferably less than 1 hour. 
   
   
       7 . Production system of tubular structures according to  claim 1 , wherein said annealing step of the preform in martensitic or austenitic-martensitic steel provides for a first step of heating from ambient temperature to the annealing temperature, a step of treatment at the annealing temperature and a step of cooling, and wherein said step of treatment at the annealing temperature is conducted at a temperatures selected in the range of from 600° C.-750° C., preferably 650° C.-700° C. 
   
   
       8 . Production system of tubular structures according to  claim 7 , wherein said annealing treatment is carried out at a temperature of about 680° C. 
   
   
       9 . Production system of tubular structures according to  7 , wherein said annealing treatment is extended for a time of at least 40 minutes, preferably at least 1 hour and preferably less than 3 hours. 
   
   
       10 . Production system of tubular structures according to  claim 9 , wherein said annealing treatment is extended for about 1 hour and 20 minutes. 
   
   
       11 . Production system of tubular structures according to  claim 8 , wherein said step of heating from ambient temperature to the annealing temperature is conducted in a time less than or equal to 1 hour. 
   
   
       12 . Production system of tubular structures according to  claim 8 , wherein said step of cooling is conducted in a time in the range of 2-4 hours. 
   
   
       13 . Production system of tubular structures according to  claim 1 , wherein said step of chemical preparation of the surfaces comprises the immersion of the tubular structure in a first tub containing an acid for a pre-established time, preferably on the order of 40 minutes, rinsing in water in a second tub and subsequent immersion in a bath of a oxalate salt for a predetermined time, preferably on the order of 20 minutes, and final immersion in a stearic ester, preferably in a concentration of 3% by weight for lubricating the external surface of the tubular structure. 
   
   
       14 . Production system of tubular structures according to  claim 13 , wherein, for the acid treatment, 140 kg/m 3  of 56% nitric acid and 40 kg/m 3  of 38-40% hydrofluoric acid are used and wherein, for the treatment with oxalate, the oxalate concentration will be generally in the range of 8-16% by weight. 
   
   
       15 . Production system of tubular structures according to  claim 2 , wherein said step of final heat treatment is conducted at a temperature in the range of 950° C.-1150° C. and for a time greater than 10 minutes and less than 1 hour. 
   
   
       16 . Production system of tubular structures according to  claim 15 , wherein said final heat treatment is followed by a step of rapid cooling and then by a step of slow cooling. 
   
   
       17 . Production system of tubular structures according to  claim 16 , wherein said step of rapid cooling is conducted by means of the contact of said tubular structure with a refrigerated, controlled gaseous atmosphere. 
   
   
       18 . Production system of tubular structures according to  claim 16 , wherein said step of rapid cooling provides for the temperature lowering of said tubular structure from about 920° C. (temperature at the furnace outlet) to about 450° C., in a time in the range of 30 seconds-2 minutes, preferably about 1 minute. 
   
   
       19 . Production system of tubular structures according to  claim 2 , wherein said tubular structure, before said final heat treatment step, is subjected to a cleaning operation for removing the lubricating residues. 
   
   
       20 . Production system of tubular structures according to  claim 19 , wherein said cleaning step is carried out by immersion in a solution of surface-active substances and carbonate salts. 
   
   
       21 . Production system of tubular structures according to  claim 1 , wherein said heat treatments of annealing, normalisation and final heat treatment are executed in controlled atmosphere furnaces to avoid that the material undergoes surface alterations, both externally and internally, and to prevent any oxidation and decarbonation. 
   
   
       22 . Production system of tubular structures according to  claim 21 , wherein said controlled atmosphere is a gaseous mixture consisting of about 50% nitrogen and about 50% reducing gas, wherein the reducing gas is preferably a gas containing hydrogen such as that obtainable by steam reforming. 
   
   
       23 . Production system of tubular structures according to  claim 1 , wherein said preform is a rough tube obtained by hot-working from a billet or other source of martensitic or austenitic-martensitic steel, preferably by extrusion. 
   
   
       24 . Production system of tubular structures according to  claim 1 , wherein said tubular structures, after said step of final heat treatment, are subjected to a step of pickling and/or passivation. 
   
   
       25 . Tubular structure in martensitic or austenitic-martensitic steel, weldable and with high mechanical strength, obtainable by means of the process according to  claim 1 . 
   
   
       26 . Tubular structure according to  claim 25 , wherein said steel is X4Cr—Ni—Mo 16-5-1. 
   
   
       27 . Tubular structure according to  claim 25 , wherein said steel has a tensile strength >1100 N/mm 2 .