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Bending steel and stainless steel tubes and pipes-Stainless steel pipe

Bending steel and stainless steel tubes and pipes-steel Pipe

Technical properties of steel tubes and pipes

The following table lists the most important properties when determining the quality of a steel tube or a stainless steel tube or pipe:

  • elongation
  • yield strength
  • tensile strength
  • rigidity
  • hardness
  • visual appeal
  • weight

 

How is a steel tube manufactured?

The commonest manufacturing process used for steel and stainless steel tubes uses coils. Coils are rolls of steel cut from a slab that has been manufactured from cast steel. During the first phase of steel tube production, the coils are cut lengthways to form wide skelps matching the diameter of the steel tube that needs to be manufactured.

 

These skelps then pass through a series of rolls. The final step is to weld the tube together along its long edge.

 

This process can be used to manufacture round, square or rectangular tube sections. With the help of matrices, various types of steel sections can be manufactured from a wide range of basic geometric shapes.

 

Boiler Steel Pipe

 

 

Types of steel tubes according to EU standards – alloys

Steel tubes, also called carbon steel tubes (AHS steels), are grouped into two main categories:

  • Mild steel: these include highly malleable and easy to work steel tubes with increased mechanical tensile and yield strength, as well as excellent impact strength
  • High-carbon steel: while these steel tubes have excellent hardness properties, they are also more brittle. High-carbon steel tubes are harder to process than mild steel tubes. They are ideal for heat treatments such as nitriding or quenching, however.

Each type of steel is given an identifier that specifies its properties.
The following table explains the meaning of these steel codes:

 
Usage: first letter of the identifying code, namely S, E or P S Structural steels
E Engineering steels
P Steel for pressure vessels
Yield strength: numerical value 235 Minimum yield point Rp=235 N/mm2
355 Minimum yield point Rp=355 N/mm2
Impact code: two letters J Testing strength > 27 J R (at 20 °C)
K Testing strength > 40 J 0 (at 0°C)
I Testing strength > 60 J from 2 (at −20 °C) to 6 (at −60 °C)
Delivery condition: letter N, M, Q or code G1, G2, G3 N Normalised
M Thermomechanically treated
Q Quenched and tempered
G1; G2; G3 Rimming steel, rimming steel not permitted, normalised rolled
Usage: letter L, S or W L Low-temperature applications
S Shipbuilding
W Resistant to atmospheric weathering

Wikipedia provides a detailed classification of steel grades according to the steel’s carbon content. In this article, we can merely state that the properties of the steel tube, such as ductility, rigidity and its general ease of processing all depend on the formulation of its alloy.

 

The properties of the steel tube also determine its suitability for a certain use or purpose, such as the production of tools or the construction of buildings, for example.

 

It’s important to note that, until 2006, almost every country 17used a different code to describe steel alloys. Only in 2006 was a standard introduced that set out a generally applicable set of specifications for steel alloys.

 

The current code used for our steel above, for example – S275JR – did not exist before 2006, with different codes being used in Germany, Italy, Spain and the United States.

  • Italy: Fe 430 B
  • Germany: St 44-2
  • Spain: AE 275B
  • United States: A 529

As you can see from the above example, the steel S355JR was referred to in Italy as FE 510 B, in Germany as St 44-2, in Spain as AE355B and in the US as A633.

 

Describing steel using the alphanumeric and numerical system

At this point, it should be noted that we’ve previously been using steel codes from the alphanumeric EU (EN) standards. However, there are also some numerical EN standards, as is shown by the following table.

 

The alphanumeric system describes the steels according to their usage, and their mechanical and physical properties. The numerical system, in contrast, defines the steel using a five-digit code number, where each digit has a separate meaning.

  • The first digit designates the material
  • The second and third digits specify the material group according to the relevant regulation
  • The fourth and fifth digits make up the sequential number for each type of material

 

Tubes made from stainless steel

Tubes made from stainless steel are rustproof, since stainless steel contains a (minimum) percentage of 10.5% chromium, which has the effect of making the steel tube corrosion-resistant.
Stainless steel includes the following types:

  • Martensitic stainless steels
  • Austenitic stainless steels
  • Ferritic stainless steels
  • Austeno-ferritic stainless steels
 

Technical properties of a stainless steel tube

The stainless steels typically used for tubes that will be bent are austentic steels, particularly:

  • AISI 304
  • AISI 316

Difference between a steel tube and a stainless steel tube

This table shows the difference in the composition of the two metal alloys of steel and stainless steel:
Metal alloy Composition
Steel Mostly iron and carbon
Stainless steel Mostly carbon, chromium and nickel

 

In contrast, the following table lists a number of differences between steel tubes and stainless steel tubes in terms of their costs, ease of processing and usage. Of particular interest are the overall costs, market price, the tube’s surface quality and corrosion resistance, and the ease of processing – especially in terms of steel pipe bending, rolling and welding.
  Steel tube Stainless steel tube
Market price – Expensive + Expensive
Weight Same Same
Surface quality Matt Glossy, reflective
Corrosion-resistant No Yes
Welding + Easy + Difficult
Typical usage Concealed tubes,
unless these have been given a special finish (e.g. coated or galvanised)		 Normally visible tubes without the need to give the tubes special treatment
 

Often, a stainless steel tube is considered to be ‘better’ on account of its corrosion resistance and mechanical properties. In reality, however, the metal alloy used for the tube – steel or stainless steel – should be chosen based on the following three factors:

  • Price
  • Weight
  • Final use or intended purpose
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