TechnologyCurrent Development ofWeather-resistant HighStrength Bolts for SteelStructures in China
中國鋼結構用耐候高強度螺栓
的發展現狀
Article by Hsien-Ming Chang
Copyright owned by Fastener World
著作權所有：惠達雜誌

Bolt connection is still irreplaceable as an importantconnection method due to its technical characteristics
such as simple construction process, easy installationand dismantling. It is widely used in plate girder bridges,
diffracted girder bridges and box girder bridges. A single
bridge uses hundreds of thousands to millions of suchbolt sets. Not only to bridges, high-strength bolts for steel
structures can be broadly applied to railways. By the end of2021, China's high-speed trains totaled a mileage exceeding
42,000 km. If we count in ordinary railroads still underconstruction and those under planning, the total mileage
reaches 150,000 km. the State Council of China printed andreleased the "Outline of National Land Planning (2016-2030)"
on February 4, 2017 which proposes to build a well-developed
and railroad network, accelerate the construction of highspeed railroads, inter-regional traffic lines, and national
railways to develop citywide/suburban railroads and optimize
the transportation network in dense urban areas; by 2030, themileage of national trains will reach more than 200,000km.
China currently uses 1,520-1,840 concrete railroad ties per
kilometer of railroad, and 4 bolt sets per railroad tie. In
minimal calculation, each kilometer of railroad uses 6,080
bolt sets, which translates to more than 600 million bolt sets
used on the whole railway network. Adding bolts in bridges
and tunnels, the total number goes up further.

China's steel structure bolts in use are mostly 8.8S and 10.9S
grade high-strength bolts and usually in compliance to GB/
T1231 standard, mostly made of ML20MnTiB, 35VB, 35CrMo,
ML40Cr and other alloy steel for manufacturing bolts, but notresistant to atmospheric corrosion. Despite having the bolt’s surface
phosphated, hot dip galvanized or Dacromet treated, if the bolt is used
for long in the countryside, the manufacturing sector, the ocean andother atmospheric environments, there are still more serious uniform
corrosion, crevice corrosion and other rust phenomena.
After installation, stress corrosion and corrosion fatigue may also
occur on ordinary high-strength bolts, and there is a risk of early
failure with sudden brittle fracture and low-stress damage, whichadds non-negligible safety hazards to steel structures such as bridges.
Although stainless steel bolts can solve the corrosion problem of
average bolts, stainless steel comes with the limitation of having
lower strength, so it can not be used to manufacture high strength
bolts for steel structures, and the research on anti-corrosion forhigh strength bolts becomes more urgent and important.

Advantages in Applying Weather
Resistant Steel Bolts
Weather resistant steel is a low alloy steel with excellent
atmospheric corrosion resistance by adding a certain amount ofCr, Ni, Cu, P, Re and other corrosion-resistant alloy elements toordinary low alloy steel. When in use, the surface of the weather
resistant steel will gradually form a layer of amorphous spineloxide layer about 50pm-100pm thick which is dense and adheres
well to the base metal. With dense and stable protection by therust layer to prevent the infiltration of H+, O2-, Cl- and otherions formed by oxygen, water, salts and acids, stops the rust andsignificantly improves resistance against atmospheric corrosion.
Weather-resistant steel can go without coating on condition of good
ventilation and drainage, alternating dry and wet environment,
low salinity, and minor acidity. Compared with the conventional"average steel structure + coating" solution, the "weather resistantsteel structure + coating-free" solution has the advantages of lower
comprehensive cost, shorter construction cycle, and effective
environmental protection, with significant economic and social
benefits. As a high-performance steel, weather resistant steel has been
widely used in overseas countries. In 1964, the U.S. applied weather惠達雜誌123

China Fastener World no.69/2023

Introduction

Technologyresistant steel to steel bridges for the first time. Now, the U.S.
weather resistant steel bridges have accounted for more than
50% of all newly built steel bridges. Many countries aroundthe world broadly use weather resistant steel on steel
bridges. Japan's weather resistant steel bridges account for
20% of all steel bridges; Canada uses weather resistantsteel on 90% of new steel bridges. The U.S. has more than
10,000 coating-free weather resistant bridges, and uses
weather resistant steel on most of the 30 new steel bridges
added every year. This has formed a system of weather
resistant steel, welding materials and high-strength bolts,
as well as their design, material selection, manufacturing,maintenance and other mature technologies and standards.

China Fastener World no.69/2023

China in the 60s and 70s of the 20th century began
in-depth research on weather resistant steel, and has
developed a variety of weather resistant steel, mainly:
0 9Cu PV R E , 0 9Cu P Ti, 0 9M n N b, 08Cu PV a nd
10CrCuSiV and so on. In recent years, the developmentof weather resistant bridge steel is fast, including
15MnCrNiCu, 35MnCrNiCu and Q345qDNHY-Ⅰ,
Q345qDNHY-Ⅱ; Q370qDNHY-Ⅰ, Q370qDNHY-Ⅱ;
Q420qDNHY-Ⅰ, Q420qDNHY-Ⅱ, and so on; Since
1991 when weather-resistant bridge steel was first used
on Beijing-Guangzhou Railway Bridge, the "weatherresistant steel + coating" solution was adopted due tothe limitations in related technology and materials.
The Shenyang Houtingxiang Bridge, the Weihe Grand
Bridge in Xianyang, and the Mutuo Bridge in Tibet
have been built. Yarlung Zangbo River Bridge isthe first authentically coating-free weather resistantdomestic steel bridge using high-strength bolts made ofweather resistant steel. Coupled with Beijing Guantang
Reservoir Grand Bridge, these two representative
coating-free weather resistant steel bridges form
the guidelines for the materials, technology andconstruction of such bridges, which marks China's
deeper and full-fledged knowledge of such bridges andtechnology, and lays the foundation for multiplyingsuch bridges.
Judging from the talks between National FastenerStandardization Technical Committee, the Bridge Steel
Structure Branch of China Steel Structure Association,
China Railway Bridge Design Institute, China Railway
Second Institute, China Railway Third Institute,
China Railway Construction Institute, Guangzhou
Municipal Institute, and Shanghai Municipal Institute,the authorities, industry associations and design
departments attach great importance to promoting
the development of coating-free weather-resistantsteel bridges in China. At present, although thereis application to large bridges, the research anddevelopment of weather resistant bolts is relatively
lagging behind, China is faced with a large numberof steel materials having corrosion failure when inuse. Particularly, bolted joints on steel structures
are much prone to corrosion in China leading toearly failure of bridges and buildings in service,
which is an urgent problem. Solving the problem ofhigh-strength bolt corrosion from the material pointof view is a critical path for the future developmentof Chinese weather resistant high-strength steel.

124

惠達雜誌

R&D of Weather Resistant Steel Bolts in China
I n 2 017, X i ng t a i I ron a nd St e el Cor p suc c e s sf u l ly t r ia lmanufactured high-strength weather resistant steel XG835NH, withthe following chemical composition: 0.32C, 0.25Si, 0.71Mn, 1.0Cr, 0.28Ni,0.28Cu, etc. A thermal simulation test determined the critical point of
the XG835NH steel as follows: Ac1 = 738℃, Ac3 = 782℃ Ac1=738℃,
Ac3=782℃, Ms=348℃. This steel is required to have atmosphericcorrosion resistance, high strength, resistance to delayed fracture
performance, and resistance to low-temperature impact in order to beused on high-strength weather resistant steel, and replace alloy bolting
steel which is used as a new steel for bridge steel connection. XG835NH
weather-resistant steel alloy has a high variety and content of chemicals;
the composition range is narrow, and at the same time there is the need forstrict control of the level of mix-up, and surface quality control in castingbillets is difficult. In a heat treatment technology test, when the cooling rateis less than 0.3℃ / s, we get a ferrite + pearlite structure; when the coolingrate is greater than 0.5℃ / s, bainite structure emerges; At the cooling rateof 10℃ / s, the structure is martensite. After heating, use quenching oilfor rapid cooling, and use high-temperature tempering to obtain tempered
sorbite plus tempered troostite. The 10.9S grade high-strength weatherresistant bolts made of XG835NH wire rods pass all performance tests
and have been used on Larin Bridge in Tibet constructed in 2019 and Yan
Chong Grand Bridge constructed for Winter Olympics 2020. The latterbridge uses weather resistant bolted joints and XG835NH weather resistantsteel products. The minimal anti-slip coefficient of the bolted joint’s
contact surface was set at a default of ≥ 0.55 upon shipping, and a minimaldefault at ≥ 0.45 upon installation. The combined use of sandblasting andwire brush meets the requirements for connecting coating-free weatherresistant steel bridges, but the post-sandblasting surface condition willchange greatly the longer the bridge is exposed in the air. The anti-slip
coefficient of is relatively stable for short-term samples; Longer outdoor
exposure will decrease anti-slip coefficient significantly, so the validity of
not requiring maintenance is being verified.
NL10.9 weatherresistant highstrength bolt steel
is a low-nickel
weather-resist ant
steel developed byAnsteel Group and isused to manufacture
high strength
weather-resist ant
bolt s for br idge s
and their nuts andwashers with anominal diameter
below φ65mm.

Larin Bridge

TechnologyT he use of h igh- streng th bolt s for connection ofsteel structures has the advantages of easy installation,
removability, non-loosening, safety, etc. The connectionof large buildings and other engineering structures
requires higher performance strength of bolts. The current
XG835NH, NL10.9 grade is a low-nickel steel, designed
to withstand atmospheric corrosion for 20 to 30 years. Inorder to meet the requirements of 80 to 100 years of marine
corrosion time, the development of high-nickel weatherresistant steel is inevitable at this stage, Ansteel Group,
Pangang and Nanjing Iron and Steel Group are working
with fastening enterprises to carry out this research.

Technical Points of Weather-Resistant
Steel Bolts
For weather-resistant steel for fasteners, steel corrosionin the atmosphere is involved in a variety of chemical,
electrochemical and physical processes occurring in the gas
phase, liquid phase and solid-phase interfaces, mostly in theform of electrochemical corrosion. Unlike stainless steel,weather-resistant steel does not form a dense, extremely thinpassivation layer, but rather, through constant interaction withthe surrounding medium, a stable rust layer is formed on thesteel surface over a period of time, which greatly delays further
corrosion of the steel.
In the marine atmosphere, the air contains a large number ofchlorine ions which deposit, resulting in steel being less resistant
to corrosion than the ordinary field atmosphere. Many domestic
and foreign scholars have studied the Ni and Cr alloy elements on
the corrosion resistance of steel in a chloride ion environment andfound that Cr is beneficial to the short-term corrosion resistanceof steel, but after a prolonged period of time, Cr has reversed
the phenomenon of corrosion resistance of steel. Therefore, inorder to ensure that the steel can have better corrosion resistancein the marine atmosphere, it is feasible to add a certain amountof Ni instead of Cr. To investigate the amount of Ni added, thecorrosion resistance pattern of steels with different Ni content
was investigated in the laboratory. China is a typical oceanic
atmosphere environment, and in response to China’s demand forcorrosion-resistant steels for the oceanic atmosphere, weather-China Fastener World no.69/2023

NL10.9 steel’s chemical composition is 0.28C, 0.15Si,0.90Mn, 0.53Cr, 0.32Ni, 0.20Mo, 0.28Cu, etc.; heat treatment
by aqueous quenching at 870 ± 10 ℃ PAG. With the increasein tempering temperature, the martensite lath form graduallydisappears. Carbides are generated from the over-saturated
α-phase, periodize on the martensitic border, and the size
increases. The 480 ± 20 ℃ tempered structure gradually change
to tempered troostite and tempered sorbite that is corrosionfree, coating-free, maintenance-free, labor-saving and energysaving, reducing environmental pollution, with better resistanceto hydrodynamic delayed fracture, meeting the technicalrequirements of 10.9S grade high-strength weather-resistantbolts. It has been successfully applied to a large number ofnational key projects, such as Larin Bridge, Hechuan River
Weather-resistant Steel Bridge, China-Russia Heihe-Helongjiang
Weather-resistant Steel Bridge, etc.

惠達雜誌125Technology
resistant nickel steel (high nickel ≥ 3% Ni and low nickel ≤ 0.5%Ni) has been developed and applied to railroad bridges. Foreignand Japanese steel companies and some research institutions did
exposure tests in different areas of different salinity for up to
11 years . It can be seen that the corrosion failure of high nickelsteel is about 70% lower than low nickel weather-resistant steel.
Many domestic and foreign studies have shown that mixup is the primary cause of pitting corrosion in low alloy steels.
Sulfides are highly likely to induce pitting corrosion comparedwith others. In the process of pitting corrosion, mixed substances
are the first to be dissolved and fall off the substrate to form pits.
Due to the embrittlement effect of P on the material, precipitates
will exist at grain boundaries, resulting in the weakening of thegrain boundaries and a decrease in the impact force. For thisreason, the P and S content of weather-resistant steel is lower
than what is required in the national standard. The sum of thetwo should be ≤ 0.025%; P and S content is lower than what isrequired in GB/T1231. In addition, weather-resistant steel issmelted by converter or electric furnace, and should be refinedoutside the furnace. This is the only way to ensure the purity ofsteel and improve corrosion resistance.

China Fastener World no.69/2023

For t he u s e of we at he r-r e si st a nt b olt s i n d i f fe r ent
envi ron ments, coati ng-f ree weat her-resista nt bolts a rerecommended for dry areas under a wild climate. Coating-freeweather-resistant steel bridges allow using blackened weatherresistant bolts; Surface-treated, Dacromet-coated, or polymer
coated weather-resistant bolts are recommended for complex
climatic conditions.

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惠達雜誌

Weather-resistant bolts exposed to atmospheric conditions will
initially form the same rust layer as plain carbon steel, but after 3
to 5a of environmental corrosion, the rust layer near the substrate
will continue to dissolve and then precipitate to form a denser
amorphous hydroxyl oxide. Finally, after many years, atmosphericcorrosion resistant steel forms a stable rust layer, which plays
a major role in protecting the internal rust layer. The layer isα-FeOOH enriched with Cr, Cu, Ni, P and other elements. Steelmaterials resistant to atmospheric corrosion is a very complex
problem associated with the medium, temperature, time, space,geography and other factors. Some of the research is now more
limited to the laboratory conclusions. There is still a lot of work tobe done in terms of phenomena, rules and mechanisms.

Conclusion
The U.S. and Japan have formed a perfect standard systemafter years of exploration, summarization and accumulation in
the construction of weather-resistant steel bridges. At present,
China promotes the application of such bridges with reference to
the American standards, such as design details, determination ofcorrosion resistance index, and so on. For weather-resistant bolted
joints, relevant national research institutes should strengthen
the basic research, determine suitable application guidelines for
China's corrosion-resistant steel bridges and weather-resistantbolts according to the climate characteristics of different regions,and long-term atmospheric-exposure-to-corrosion test should becarried out to accumulate more fundamental information for theimprovement of the standardized system, going from Made in
China to Innovation by China.