By M. Denham
Chairman & Managing Director
Elland Steel Structures Ltd
20th June 2017
Structural steels are identified by the leading S before say S355, they are generally easily weldable. Engineering steels, ones with the leading letter, E, are not easily weldable, they generally have far too much carbon in them which leads them to be far too hard.
355 is in MPa, where 1 MPa = 1 N/mm2 and is the nominal yield strength for thickness up to 16mm.
The first thing to look for is to make sure you have a CE marked product, which is mandatory in the UK. Check to see that the test certificate is a 3.1 in accordance with EN 10204. If marketed in the UK, the test certificate should be in English. On the test certificate there will be information on the section size, grade, sub-grade, the chemical composition, Charpy values, nominal yield strength, tensile strength, elongation and CEV. There may be more than one section tested on one certificate, so sometimes we need to rely on batch traceability. 3.1 test certificates are the most common requirement for UBs and UCs, but there are options for 3.2 test certificates, which imply third party testing. In practice however, even if you specify a 3.2 test certificate you are not probably going to get the level of testing you want as the independent assessors will not be in attendance for the duration and there is a level of trust awarded to good steel producers such as British Steel.
The sub-grade, JR, J0 etc is a measure of the toughness of the steel, typically measured using a Charpy impact test. In the list above, JR is the least tough material and K2 is the toughest. The more Joules are measured at the same test temperature the tougher the material and the lower the test temperature the same material will demonstrate a lower Charpy impact value. K2 is listed above as having 40J at -200C, but this is equivalent to 27J at -300C.
Designers ideally want the steel to be ductile for use in elastic-plastic designs, the ductility is a measure of the toughness of the material. The toughness is effected by the chemical composition of the steel and the grain size of the micro-structure. The smaller the grain size the better the steel in relation to toughness. In fabrication if the material is heated above a certain point, grain growth can occur and the steel once it has cooled will be less tough. During weld procedure testing, there is a requirement to test the steel after welding to ensure the welding process has not affected the parent metal too much. Adding Nickel to the chemical composition will help to increase the toughness of the material at low temperatures.
The Vickers hardness test is a method of measuring the hardness of the material, the usual limit is 380Hv for steel grade S235 to S460, refer to Table 10 of BS EN 1090-2.
If the material exceeds this limit there is a danger that the material will be too hard and the material will crack, with fatigue sensitive loading this could lead to failure.
The table above is an extract from Table 2 of BS EN 10025-2 for the ladle analysis (heat or cast analysis) and is a measure of the composition of elements, the numbers in the table are the maximum limits of the element for the particular grade and sub-grade.
Table 2 of BS EN 10020:2000 gives the boundary between non-alloy and alloy steels by ladle analysis. For example if the amount of Aluminium in the steel was greater than 0.3% then the steel is said to be an alloy.
The carbon equivalent value (CEV) is one of the measures of how weldable a steel is, the higher this value is the harder the steel is to weld. The equation above is the one adopted by the UK in particular and most of Europe, there are other versions used for differing purposes in different parts of the world.
The CEV is used in conjunction with the combined thickness, the hydrogen content of the welding consumable and the expected heat input of the welding process to calculate the requirement for any preheat prior to welding in accordance with BS EN 1011-2.
Where the Consulting Engineer is expecting some through-thickness shrinkage or residual stresses occurring, say in T-Butt welds they may require a STRA test to check for resistance to lamellar tearing. The test specimen is subjected to tensile load, above the yield point the steel will start to reduce in area in the work hardening phase, eventually failing the net reduction of area at failure is measured. For a material to be considered as say Z35, the steel must be able to resist tension to such an extent that the reduction of the area is 35% of the original area. There are other Z grades, Z15 and Z25, but these are never rolled unless large quantities are required.
The customer is allowed to select all kinds of options when ordering to BS EN 10025, however in reality unless bulk mill orders of the section are required you will not be able to select any options. The majority of steelwork contractors buy their steel from stockholders and as such no further options are available at time of order.