Fabrication and welding of High Strength Low Alloy (HSLA) Steels

Alloy steels known as high-strength low-alloy (HSLA) provide increased strength-to-weight ratios over conventional low-carbon steels for only a modest price premium. Because HSLA steels are stronger, they can be used in thinner sections, making them particularly attractive for transportation-equipment, components where weight reduction is important. HSLA steels are available in all standard wrought forms – sheet, strip, plate, structural shapes, bar-size shapes, and special shapes.

 Typically, HSLA steels are low-carbon steels with carbon content between 0.05–0.25% to retain formability and weldability and up to 1.5% manganese, strengthened by small additions of elements, such as columbium, copper, vanadium or titanium and sometimes by special rolling and cooling techniques. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-perlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic volume fraction, yet maintains and increases the material’s strength by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Precipitation strength also plays a minor role.

 Zirconium, calcium, and rare earth elements are added for sulfide-inclusion shape control which increases formability. These are needed because most HSLA steels have “direction sensitive” properties.

 There are two ways such steel of high strength and high toughness steels are produced,

1                    By micro-alloying: Adding small amount of strong carbide and nitride formers and

2          By very careful control of the rolling temperature and there by controlling rolling or in other words thermo-mechanically controlled process (TMCP)  

 The highest strength is achieved by a combination of the two methods. The aim of both methods is to produce as small a gain size as possible, fine grain gives the best notch toughness properties.

 The additional benefit is the increased weldability of the material. This is achieved by reducing the hardenability of steel; the carbon content of some steel may be as low as 0.05% and reduced presence of elements like Sulphur and phosphorous.

To compensate for the loss of carbon and to increase tensile strength small amount of niobium (<0.10%), titanium (<0.03%), and vanadium (<0.15%) are added. In some grades of steel, strong carbide and nitride forming elements like molybdenum, chromium, copper and nitrogen are also added for similar effect. These formations allow fine dispersion of stable precipitates that inhibit grain growth during hot-rolling process, and assist in nucleating fine-grained ferrite during cooling process. Possibility of precipitation hardening is also an added advantage of these elements.

 TMCP is also used for similar effect of grain refinement leading to increased strength and toughness. TMCP is ordinarily carried out at about 850^oC to 900^oC (1562^oF to 1652^oF), this allows for elongated crystalline austenitic grains. Accelerated cooling from the rolling temperature develops very fine-grained ferrite on the boundaries of the austenite grains.              

Improved-formability of HSLA steels is achieved by addition of such elements as zirconium, calcium, or rare earth for sulfide-inclusion shape control. This is an important property to consider when designing because due to the higher strength of the material the parts made from HSLA steels would be relatively thinner in cross sections as compared to the parts made from low-carbon steel.

HSLA steel Fabrication
Formability and impact strength can vary significantly when tested longitudinally and transversely to the grain. Bends that are parallel to the longitudinal grain are more likely to crack around the outer edge because it experiences tensile loads. This directional characteristic is substantially reduced in HSLA steels that have been treated for sulfide shape control.

 Grades known as “improved-formability” HSLA steels (sheet-steel grades designated ASTM A715, and plates designated ASTM A656) have yield strengths up to 80,000 psi, yet cost only about 20% more than a typical 35,000-psi yield strength plain-carbon steel.

Steel grades in ASTM A690 are noted for corrosion resistance in seawater splash zones so is used in docks, seawalls, bulkheads and related uses.

HSLA steel welding

Despite improved weldability discussed above these steels have some fabrication challenges one of them being Hydrogen Induced Cracking (HIC).

1. The low carbon content in these steels reduces the CE to as low as 0.3 or even lower. This translates into steels having very low tolerance for hydrogen leading to cold cracking. The IIW formula for calculating CE to establish pre heat is no more valid for these steels.

These steels therefore can be welded at significantly lower pre heat temperature; this is one significant advantage over the normal carbon steel.

The highest potential for cracking of welds in these steels is not in HAZ but in the weld metal. Some of the key reasons for this phenomenon are;

• The high strength of parent metal develops higher residual stress during welding.
• To match the tensile strength and toughness of the parent metal the filler metal needs to be more highly alloyed and therefore will have higher CE. Filler metal CE of up to 0.6 (IIW) is often used. (Example, 700 MPa YS with E 11018 G electrode)
• The weld metal transforms from austenite to ferrite at a lower temperature than the parent metal. (A reveres of the transformation in normal carbon steel).
• As a result of the point made above, the hydrogen in HAZ is rejected into the still austenitic weld metal, which has high solubility for hydrogen. A pre heat based on the weld metal composition is therefore advised and low hydrogen technique must be used. Only exception to this is to allow cellulosic weld in root and hot pass in pipeline welds if manual (SMAW) process is used, and rest of the weld has to be low hydrogen process. In Higher grades it is strongly recommended that proven automatic welding process using low hydrogen consumable is used.

1. Even though these steel contain low Sulphur, the lower carbon percentage may suffer from solidification cracking in root pass, particularly if the welding speed is relatively high. This is particularly because the high dilution of filler metal produces a weld deposit that is low in carbon, that allows for excessive grain growth of austenite during welding and these large grains increase the risk of center line solidification cracking especially in the root bead. This is a very common issue with pipeline welding that use cellulosic electrodes, one reason for that would be the infusion of hydrogen in the weld coupled with the speed of deposition associated with down hand welding technique.
2.  The toughness and strength in HAZ can be a serious issue. The steel is manufactured with great control as far as TMCP process is concerned. Contratsting with that is the uncontrolled cycles of heating and cooling associated with welding. The microstructure of HAZ will vary with respect to the composition of steel and welding process heat input. A high heat input will promote grain growth and this will have an adverse effect on both strength and toughness. As guidance only, the heat input must be controlled to about 2.5 kJ/mm or lower some alloys with titanium and boron can tolerate heat input up to 4.5kJ/ mm. The interpass temperature should be maintained at 250^oC maximum.  

 These steel are not recommended to be heat treated after welding. Post weld heat treatment may be required if thickness is in excess of 35 mm, great care is to be exercised in determining the need and process of PWHT.  

ASTM A514 and A514M-05 high-strength, low-alloy, quenched-and-tempered steels are among the HSLA materials. Welding these materials successfully is a matter of understanding some key factors, including filler metal choices and preheating and interpass heat requirements.

ASTM A514 material specification has several grades (Grade A, B, E, F, H, P, Q, and S). Each of these grades have a unique chemistry and the chemistry within a grade further differs with the thickness to which it is rolled, from 1.25 in. to 6 in.

•  The material thickness affects the mechanical properties. For instance, A514 rolled to 2.5 in. or less is required to have 110,000-psi to 130,000-psi tensile strength, 100-KSI minimum yield strength, and 18 percent elongation.
•  For materials thickness 2.5 to 6 in., the mechanical properties are 100-KSI to 130-KSI tensile strength, 90-KSI minimum yield strength, and 16 percent elongation.
• The specified hardness for material thickness up to and including 0.75 in. is 235 to 293 HBW (Brinell). Note that the specification does not list hardness requirements for materials thicker than 0.75 in.

One of the reasons for the difference in properties among these thicknesses is the quenching effect. The thicker the material, slower the quench rate, which results in lower minimum yield and tensile strengths.

Typically, this material is used for structural applications. In many cases, the term structural refers to buildings, but the material also is used in heavy equipment structures to reduce weight and improve payload capacity, such as in railcars and their components, large mining truck frames, semi trailer frames, and crane boom sections etc.

Because the typical hardness of the materials is 22 to 27 Rockwell C, (about 248 to 279 Vickers 10 kg scale) the material is also used for wear strips, cutting edges, and side cutters. Typical applications are backhoe buckets and other wear components in earthmoving equipment.

 Filler Metal selections:

 Welding ASTM A514 is not complicated when some precautions, especially with selection of filler metal are used.

 A primary concern is filler metal hydrogen content. Filler metals that deposit weld metal with diffusible hydrogen content greater than 8 ml per 100 grams of deposited weld metal should not be used. ASTM A514 is sensitive to diffusible hydrogen, which may result in hydrogen cracking.

Table -1: Typical Filler Metal Selection
ASTM A514 and A514M-05 up to 2.5 Inches Thick

 

Welding Process	AWS A5 Specification	Classification
SMAW	A5.5/A5.5M-2005	E11018M or E12018M
GMAW	A5.28/A5.28M-2005	Solid electrodes ER 110S-1 and ER120S-1 or Metal-cored electrodes E110C-K3, E110C-K4, and E120C-K4
FCAW	A5.29/A5.29M-2005	E11XT1-K3C and -K3M E11XT5-K3C and -K3M E11XT5-K4C and -K4M E12XT5-K4C and -K4M
SAW	A5.23/A5.23M-1997	Solid electrode/flux combination F11AX-EXXX-XXX and F12AX-EXXX-XXX or Composite electrode/flux combination F11AX-ECXXX-XXX and F12AX-ECXXX-XXX

 

 

The filler metal strength depends on the application of the A514. Table –1 shows filler metals that can be used to match the strength of base materials up to 2.5 in. thick. This list is suitable for selection of filler metal where the same mechanical properties of weld metal as that of base material are required. On base material thicknesses greater than 2.5 in., same filler metals can be used if the overmatching strength of weld metal properties meets the design requirements.

 Table –2: Filler Metal Selection
ASTM A514 and A514M-05 for > 2.5 Inches thick steel

 

Welding Process	AWS A5 Specification	Classification
SMAW	A5.5/A5.5M-2005	E10018M
GMAW	A5.28/A5.28M-2005	Solid electrodes ER 100S-1 or Metal-cored electrodes E100C-K3
FCAW	A5.29/A5.29M-2005	FCAW A5.29/A5.29M-2005 E10XT1-K3C and -K3M E10XT5-K3C and -K3M E10XT1-K7C and -K7M E10XT1-K9C and -K9M
SAW	A5.23/A5.23M-1997	Solid electrode/flux combination F10AX-EXXX-XXX or Composite electrode/ flux combination F10AX-ECXXX-XXX

 

When joining A514 to other low-alloy steels or carbon steels of lower strength, use a filler metal with strength that meets the lower-strength base material’s properties.

The focus should be on the hydrogen cracking risk, not just the strength of the weld.

Heat Input Control

Even though A514 is readily weldable, excessive preheat and interpass temperatures and welding heat input can affect the alloy’s chemical properties.

 Table –3lists typical preheat and interpass temperatures for A514. These temperatures apply whether the weld is on A514 to itself or to other, lower-strength materials.

 Table –3 Typical Pre-Heat and Interpass temperatures

 

Thickness (In.)	Maximum Preheat and Interpass Temperature (F)	Maximum Preheat and Interpass Temperature (F)
Up to 0.75	50	400
0.75 – 1.5	125	400
1.5 – 2.5	175	400
> 2.5	225	400

 

It should be noted that preheat and interpass temperatures higher than those shown in Table- 3 may alter the mechanical properties of the material. Tempil® Sticks, contact pyrometers, infrared thermometers, or other heat-measuring devices may be used to control preheat and interpass temperatures.

 In addition to the preheat and interpass temperature controls, heat input, which is a function of amperage, voltage, and travel speed, must be restricted. Heat input is expressed in joules per inch. The formula is:

 Heat Input (joules/in.) = (Amperage * Voltage* 60) ÷ Travel Speed (IPM)

 Typical heat input is about 55,000 joules per in. (±20 percent). Various steel grades will have different range of heat inputs, it is advised to contact the steel manufacturer for recommendations.

 As a final precaution, ASTM A514 is not intended to be used in the postweld heat-treated (PWHT) condition, as it will alter the mechanical properties for which the material was intended. Other sources of information for welding A514/A514M-05 steel are the steel manufacturers’ fabrication guides and:

•  AWS D1.1, Structural Code—Steel, AWS D14.3,
• Specification for Welding the Earthmoving, Construction, and Agricultural Equipment,
• AWS D15.1, Railroad Welding Specification— Cars and Locomotives.

 Corrosion:

HSLA steels are also more resistant to rust than most carbon steels, this property can be ascribed due to the lack of pearlite in this type of steel. Pearlite is a fine layer of ferrite (almost pure iron) and cementite.

 

While additions of elements such as copper, silicon, nickel, chromium, and phosphorus can improve atmospheric corrosion resistance of these alloys, they also increase cost. Galvanizing, zinc-rich coatings, and other rust-preventive finishes are used to protect HSLA-steel parts from corrosion.