Requirements for Stainless Steel Rebar Presented by Suresh Gupta ASTM Committee Meeting A0105 May 8 2012 Purpose of Elongation Elongation is a measure of ductility which provides confidence that metals can be ID: 273554
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Slide1
Elongation
Requirements
for
Stainless Steel
Rebar
Presented by:
Suresh Gupta
ASTM Committee Meeting A01.05 May 8, 2012Slide2
Purpose of Elongation
Elongation
is a measure of ductility, which provides confidence that metals can be
formed,
without cracking or fracturing
.
In
rebar,
it defines ductility so that bars can be bent without cracking.
I
f bars do not crack during fabrication, any excess elongation is irrelevant
.Slide3
Alloy Characteristics
With
all other variables remaining same
,
Austenitic Alloys have highest elongation.
Duplex Alloys have elongation in mid range
Ferritic Alloys have lowest elongationSlide4
Measurement Methods and Facts
% Elongation, when measured
in
a fixed gage length, e.g. 8 inches,
is lower than when measured in a gage length, as a multiple of
diameter,
e.g.
4 X diameter (4d
).
(when
4d is less than
fixed gage length)
.
Elongation is generally lower as diameter goes smaller
.
This fact is clearly described in ASTM E8, Appendix XISlide5
ASTM E8
Standard Test Method for Tension Testing of Metallic
Materials
Appendix
XI – Factors Affecting Tensile Test Results
Comparison of elongation values determined using different specimens requires that the following ratio be controlled:
L
o
/(A
o
)
1/2
(XI.1)
Where:
L
o
= original gage length of specimen, and
A
o
= original cross-sectional area of specimen.
XI.4.4.1 Specimens with smaller L
o
/(A
o
)
1/2
ratios generally give greater elongation.
Note:
(A
o
)
1/2
is proportional to diameter (d).
Therefore, for same ductility, larger diameter samples result in greater elongation values, when measured in fixed gage length.Slide6Slide7Slide8Slide9
ASTM A955-9a
Until revision 10a, elongation was not required for
sizes #
3-5
Grade 75, probably
because it was known that it is unrealistic or not necessary.
Grade 40 for sizes 7 and larger because, in any condition it will meet the requirement, anyway.Slide10
% Elongation Requirements
for Rebar
Carbon
and Alloy steels
ASTM A615 requires 6 to 7
%
for Grade 75
ASTM A706 requires 10 to 14% for all Grades.
Stainless
Steels
BS 6744 requires 14% minimum in
5d gage length
for Grade 500 (72.5 ksi
).
ASTM A955 requires 20% minimum in
8 inch gage length
for all grades, all sizes.Slide11
Manufacturing
&
Process Control
Even with precise chemistry and process control,
duplex stainless steels
meet the elongation requirement very marginally.
If rebar is supplied in coils, machine straightening will cause work hardening and elongation will go down even further.
This may cause unnecessary dispute.Slide12
Rationale
If 6% elongation in carbon steel is sufficient
If
14
% in
5d
is sufficient for stainless steel rebar per BS6744 in Europe,
It should be sufficient for us as well.
FYI
ASTM Specifications for plain smooth bars require elongation in 4dSlide13
Proposal
It is therefore recommended that,
Using a foot note to Table 3, elongation requirements be lowered to 14% for Grade 75,
for all sizes of duplex grades and,
all grades of stainless steels under 1.00 inch.