High Performance Steel Designers' Guide
2.0 Material Properties
The AASHTO Subcommittee on Bridges and Structures, and the AASHTO T-14 Technical Committee for Structural Steel Design continues to review and adopt new specifications as the research and development of HPS progresses. They have modified the AASHTO LRFD Section 6.4.1 to include ASTM A709 Grade HPS 70W as a replacement of AASHTO M270 Grade 70W.
The chemistry for HPS 70W (HPS 485W) and HPS 50W (HPS 345W) is shown in the following table:
|Old 70W *||Min.||-||0.8||-||-||0.25||0.2||-||0.4||-||0.02||-|
|HPS 70W & HPS 50W||Min.||-||1.1||-||-||0.3||0.25||0.25||0.45||0.02||0.04||0.01|
* The conventional ASTM and AASHTO 70W grade steel has been replaced by HPS 70W grade steel.
HPS 70W is produced by quenching and tempering (Q&T) or Thermal-Mechanical Controlled Processing (TMCP). Because the Q&T processing limits plate lengths to 50 ft. (15.2 m) in the U.S., TMCP practices have been developed to produce HPS 70W up to 2 inches (50 mm) thick and to 125 feet (38 m) long, depending on the weight.
The chemistry for HPS 50W grade steel is the same as HPS 70W shown in the table above. ASTM A709 Grade HPS 50W is contained in A709-01 and is produced using conventional hot-rolling or controlled rolling up to 4" thick in lengths similar to Grade 50W steel.
|HPS 50W Up to 4" As-Rolled||HPS 70W Up 4" (Q&T). 2" (TMCP)|
|Yield Strength, Fy, ksi (MPa) min.||50 (345)||70 (485)|
|Ultimate Tensile Strength, Fuksi (MPa)||70 (485)||85-110 (585-760)|
|CVN, minimum*Longitudinal orientation||25 ft.-lbs. (41 J)@ 10°F (-12°C)||30 ft.-lbs. (48 J)@ -10°F (-23°C)|
* One of the goals of the research program is to develop HPS with CVN toughness meeting the requirements of Zone 3. CVN tests show that HPS have CVN toughness far exceeding these minimum levels. Specification minimum requirements for fracture critical designs are 5 ft-lbs higher than the values shown in Table 2.2.1.
The fatigue resistance of high performance steels is controlled by the welded details of the connections and the stress range, as is the case for conventional steels. The fatigue resistance is not affected by the type and strength of steels. Tests on high performance steel conclude that the fatigue categories given in the AASHTO LRFD, Section 6.6.1 Fatigue also apply to high performance steel welded details.
The fracture toughness of high performance steels is much higher than the conventional bridge steels. This is evident from Figure 2.3.1, which shows the Charpy V-Notch (CVN) transition curves for HPS 70W(HPS 485W)and conventional AASHTO M270 Grade 50W steel. The brittle-ductile transition of HPS occurs at a much lower temperature than conventional Grade 50W steel. This means that HPS 70W(HPS 485W) remains fully ductile at lower temperatures where conventional Grade 50W steel begins to show brittle behavior.
|TF °F = 1.8 (TC °C) + 32 |
1 ft.-lb. = 0.729 J
Fig. 2.3.1 CVN Transition Curve 
The current AASHTO CVN toughness requirements are specified to avoid brittle failure in steel bridges above the lowest anticipated service temperature. The service temperatures are divided into three zones as shown in Table 2.3.1 below.
|Minimum Service Temperature||Temperature Zone|
|0°F and above||1|
|Below 0° to -30°F||2|
|Below -30°F to -60°F||3|
The AASHTO CVN requirements for these zones are shown in Table 6.6.2-2 Fracture Toughness Requirements in the AASHTO LRFD. The HPS 70W(485W) steels tested so far show ductile behavior at the extreme service temperature of -60°F for Zone 3. It is a major accomplishment of the HPS research and an important advantage of HPS in controlling brittle fracture.
With higher fracture toughness, high performance steels have much higher crack tolerance than conventional grade steels. Full-scale fatigue and fracture tests of I-girders fabricated of HPS 70W (485W) in the laboratory showed that the girders were able to resist the full design overload with fracture even when the crack was large enough to cause 50% of loss in net section of the tension flange . Large crack tolerance increases the time for detecting and repairing fatigue cracks before the bridge becomes unsafe.
A main thrust of the HPS Research Program is to develop bridge steels with significantly improved weldability . Improving weldability reduces the high cost of fabrication associated high preheat temperatures, heat input control, post-weld treatment, and other stringent controls, and to eliminate hydrogen induced cracking in the weldment.
Hydrogen induced cracking, also known as delayed cracking or cold cracking, has been one of the most common and serious problems encountered in steel weldments in bridges. The common source of hydrogen is from moisture. Grease, oxides and other contaminants are also potential sources of hydrogen. Hydrogen from these sources can be introduced into the weld region through the welding electrode, shielding materials, base metal surface and the atmosphere.
Hydrogen-induced cracking can occur in the weld heat affected zone (HAZ) and in the fusion zone (FZ). While the reasons for cracking are the same, controlling the factors that cause cracking can be different for the HAZ and FZ. For the HAZ, control of cracking comes from the modern steel-making processes, which incorporate means to avoid susceptible microstructures and eliminate sources of hydrogen in the base metal (steel) and using proper welding techniques, including preheat and heat input. For the FZ, control of susceptibility to hydrogen-induced cracking is achieved by adding alloying elements in the consumables, and using proper welding techniques, including preheat and heat input.
The most common and effective method of eliminating hydrogen-induced cracking is specifying minimum preheat and interpass temperature for welding. In general, the higher the preheat the less chance for formation of brittle microstructures and more time for the hydrogen to diffuse from the weld. However, preheating is time consuming and costly. One of the goals in developing high performance steels is to reduce or eliminate preheat. This goal has been successfully accomplished as shown in Table 2.4-1 below:
|To ¾"||Over ¾" to 1 ½"||Over 1 ½" to 2 ½"||Over 2 ½"|
|Grade 70W||50°F (10°C)||125°F (52°C)||175°F (79°C)||225°F (107°C)|
|HPS 70W||50°F (10°C)||70°F (21°C)||70°F (21°C)||125°F (52°C)|
* Denotes the level of hydrogen measured in the laboratory in terms of milliliter per 100 grams of deposited weld metal, e.g. H4 means 4 ml/100g of diffusible hydrogen in the weld metal.
Minimum preheat for HPS 50W has not yet been established. It is the subject of ongoing research. The conservative approach is to specify the same preheat requirements as for M270 Grade 50W. On the other hand, the chemistry for HPS 50W is the same as for HPS 70W, it is reasonable to expect that the welding procedures for HPS 50W will be somewhat less stringent. In general, the AWS D1.5 Bridge Welding Code can be used for the fabrication of HPS 50W steel. However, until research results and fabrication experiences on the weldability of HPS 50W are available, the designers should specify weld procedures and qualification tests on a project-by-project basis.
The AASHTO HPS Guide and AWS Code contain supplementary welding provisions applicable to HPS. The designers should make sure that the applicable provisions of these two documents are made a part of the contract documents. Some key elements pertaining to welding of HPS are:
- The use of only low-hydrogen practices when reduced preheat is to be used.
- Only submerged arc (SAW) and shield metal arc (SMAW) welding processes are recommended for HPS. Research is ongoing for the use of gas metal arc (GMAW) welding process.
- The diffusible hydrogen level is limited to a maximum of 8 mL/100g (H8). SAW consumables should be handled such that the diffusible hydrogen is controlled to a level of H4 maximum if reduced preheat is to be used. SMAW consumables may meet level H4 or H8.
- Consumables with matching weld strength are recommended for SAW complete penetration groove welds connecting Grade HPS 70W plates. Consumables with undermatched weld strength are strongly recommended for all fillet welding. The designers should specify on the contract drawings or special provisions where undermatched fillet welding is permitted or required.
- For connecting HPS 70W to Grade 50W, consumables satisfactory for Grade 50W base metals are considered 'matching' strength. However, it is recommended that the diffusible hydrogen level be limited to H4 or H8.
- SAW consumable combination of Lincoln LA85 electrode and MIL800HPNi flux consistently produces acceptable quality weld metal. This applies to both Q&T and TMCP products.
- For first-time fabrication of HPS, it is beneficial to perform weld procedure qualification tests on mock-ups of HPS butt welds and HPS to Grade 50W butt and fillet welds using consumable combination proposed for the production welds prior to starting fabrication.
- Improved weldability still needs care and good workmanship to produce quality welds.
- The AISI Website (www.steel.org/infrastructure/bridges) contains a wealth of information on HPS and updates to the AASHTO HPS Guide.
- The cost effectiveness of HPS has been demonstrated by the design and construction of HPS bridges by many states.
It was part of the initial research objective to develop HPS with "weathering characteristic", meaning HPS should have the ability to perform without painting under normal atmospheric conditions. HPS steels have slightly better atmospheric corrosion resistance than the conventional grade 50W or 70W steels. For example, as measured in accordance with ASTM G101, the atmospheric corrosion resistance index (CI) for conventional Grade 70W is 6.0, while the index for HPS 70W is 6.5. Long-term atmospheric corrosion tests are underway to further support this projection.
The designers should follow the same guidelines and detailing practice for conventional weathering grade steels to assure successful applications of HPS steels in the unpainted conditions. Guidelines for proper application of unpainted weathering steels in highway bridges may be found in the FHWA Technical Advisory T 5140.22, Uncoated Weathering Steel in Structures, dated October 3, 1989.