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Materials
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Table 1 . Typical chemical composition of selected duplex and austenitic stainless steel alloys .
Duplex
Type EN ASTM / ASME Typical composition (%)
No Type UNS C Cr Ni Mo N Other
PRE *
Forta LDX 2101 |
1.4162 |
S32101 |
0.03 |
21.5 |
1.5 |
0.3 |
0.22 |
5Mn Cu |
26 |
Forta FDX 27 |
1.4637 |
S82031 |
≤0.04 |
19.0-22.0 |
2.0-4.0 |
0.6-1.4 |
0.14-0.24 |
≤2.5Mn |
27 |
Forta DX 2205 |
1.4462 |
2205 |
S32205 |
0.02 |
22.0 |
5.7 |
3.1 |
0.17 |
|
35 |
Forta SDX 2507 |
1.4410 |
2507 |
S32750 |
0.02 |
25.0 |
7.0 |
4.0 |
0.27 |
|
43 |
Austenitic Core 304L / 4307 |
1.4307 |
304L |
S30403 |
0.02 |
18.1 |
8.1 |
– |
– |
|
18 |
Supra 316L / 4404 |
1.4404 |
316L |
S31603 |
0.02 |
17.2 |
10.1 |
2.1 |
– |
|
24 |
Ultra 904L |
1.4539 |
904L |
N08904 |
0.01 |
20.0 |
25.0 |
4.3 |
– |
1.5Cu |
34 |
Ultra 254 SMO |
1.4547 |
|
S31254 |
0.01 |
20.0 |
18.0 |
6.1 |
0.20 |
Cu |
43 |
* PRE = % Cr + 3.3x % Mo + 16x % N |
fracture elongation of about 25 % vs . 40 % for most austenitic grades , which clearly indicates the lower formability of the former . More ductile duplex alloys such as Forta FDX 27 ( 1.4637 , S82031 ) utilize the so-called TRIP effect , induced by its metastable microstructure , to further improve their formability to a level which approaches that of austenitic alloys . However , the corrosion resistance of this grade is comparable to that of type 316 and therefore is not always resistant enough for the harsh conditions encountered in the refining and petrochemical industries . The most critical process parameters to consider are the higher content of impurities ( e . g ., chlorides and sulfur compounds ), reduction of pH , or higher temperatures . Therefore , when harsher environments encountered , the question boils down to asking if conventional DX 2205 , or even super-duplex SDX 2507 , can serve as an alternative to the more expensive high-alloy austenitic grades ? The typical chemical compositions of the selected duplex and austenitic stainless steels are depicted in Table 1 , with a comparison of their critical material properties in Fig . 1 and 2 .
Formability and Forming Limit Diagram ( FLD ) A better tool to assess the formability the complex metal sheet rather than just looking at elongation values is to consider a forming limit diagram ( FLD ). In this type of diagram , different bi-axial strain conditions can be evaluated . The three major forming conditions are : plane strain , stretch forming , and deep drawing , depicted in Fig . 3 . For PHE sheets there are different types of strain conditions depending on the specific shape of the pattern , the radii , and height of the corrugations , which influence the flow and heat transfer capacity . The shape complexity has a strong impact on how the tool and the forming operation can be performed using a certain type of alloy . The graph in Fig . 4 . displays the typical forming limit curves ( FLCs ) for three different types of stainless steels : ferritic , austenitic , and ferritic-austenitic ( duplex ). As expected , the austenitic type provides the best formability considering all strain conditions , and the ferritic type the worst even though the latter , in relative terms , performs better in deep drawing . The duplex type with its dual microstructure sits in-between [ 1 ] .
Fig . 3 . FLD at different forming conditions . Fig . 4 . Typical FLCs for different types of stainless steel alloys .
32 Heat Exchanger World November 2022