Top Chinese supplier of wind turbine founation anchor bolt.

CIEEMA Visited Longteng High Strength Steel Production Line

CIEEMA Visited Longteng

On May 12, the executive vice president and secretary general of CIEEMA, Han Jixian, chief engineer and Wang Liting, honorary chairman of Nanjing Tongtian Rock Cotton Co., Ltd., and other delegations were invited to Henan Yulong Group for investigation and guidance.

Anchor Bolt Production Line

Under the guidance and introduction of Chairman Feng Dibao, the CIEEMA ‘s research team focused on Anyang Longteng Heat treatment Co.,Ltd, and visited wind energy anchor bolt production line, anchor plate production line, steel tie rod production line and related product display areas in turn.

Outlook of Company Future

In the conference room of group company, research team exchanged and discussed with the leaders of Yulong Group, person in charge of each sector and experts.Secretary-General Han Jixian first expressed gratitude to Chairman Feng and other leaders for their warm reception on behalf of the research team of the Association!

It was stated that this was the last time that Chairman Feng had been invited for many times, and it was also the first visit and investigation organized by Association after the epidemic was relieved. At the same time, he took this opportunity to learn about the industry’s development status, difficulties and demands during the epidemic.

Common Anti-corrosion Methods for Mining Rock Bolt

Self Drilling Resin Rock Bolt

Various types of anti-corrosion methods for mining rock bolts are being developed. As mining rock bolts are made of steel, if they are stored and pretreated improperly, once rust occurs, it will lead to serious accidents with loss of both human lives and treatures. Usually it is started with corrosion and scrap. Therefore, anti-corrosion treatment measures for mining rock bolt are very necessary. 

The problem of corrosion and cracking of rock bolts in coal mines and other underground mines, and high variability of mine water corrosivity in mining industry, it has been noted by late corrosion study. The use of higher-tensile steels for rock bolts and anchor bolts, presents a risk for a greater incidence of corrosion, which is dangerous in underground environments. There are many other corrosion issues that need to be taken care of but are not included here.

Rock Bolt Corrosion Situation

The problem of corrosion and cracking of rock bolts in coal mines and other underground mines, and high variability of mine water corrosivity in mining industry, it has been noted by late corrosion study. The use of higher-tensile steels for rock bolts and anchor bolts, presents a risk for a greater incidence of corrosion, which is dangerous in underground environments. There are many other corrosion issues that need to be taken care of but are not included here.

Common Anti-corrosion Methods for Mining Rock Bolts

  • Brush coating method:
    Coating is a widely used anti-corrosion method, and it is an indispensable anti-corrosion material and identification mark on mining anchor rock bolt products. Paint is also a non-metallic material. It is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., and covers metal surface to isolate medium from atmosphere to achieve anticorrosion.
  • Metal Surface Treatment:
    Metal surface treatment processes include sleeping coatings, surface penetration, and surface oxidation and passivation. Its purpose is to improve corrosion resistance and mechanical capabilities of metal.
  • Add corrosion inhibitor:
    Adding a small amount of other special substances to corrosive medium and corrosive substances can greatly slow down corrosion rate. This special substance is called corrosion inhibitor. Corrosion inhibitor is always consist of sodium nitrite and sodium chromate, which can form a passivation film on anchor bolt surface. The solvent can slowly dissolve corrosion inhibitor and can also act as a lubricant. Zinc powder is added as a sacrificial metal. In fact, zinc is also a corrosion inhibitor. It can first combine with chloride in asbestos, so as to reduce chance of contact between chloride and rock bolt, thereby achieving purpose of anticorrosion. If corrosion inhibitors such as red lead and calcium lead acid are added to coating, spraying on the surface can prevent atmospheric corrosion.

Self Drilling Mining Anchor-Rock Bolt

Mining Self Drilling Rock Bolt

Self drilling mining anchor – rock bolt is one of the important support part in coal mines. Its existence solves many problems in construction and can ensure grouting effect under the chaotic geological conditions.

Mining Rock Bolt Installation Method

Mine anchor – rock bolt is used as a tensile member deep into the stratum. One end is connected to engineering structure and other end is deep into stratum. The entire anchor section is divided into a free segment and an anchor segment. In free segment, tensile force at anchor head is transmitted, The anchor function area is to apply prestress to anchor rod; anchoring section refers to area where cement slurry binds prestressed tendons to soil layer. Its function is to increase conflict effect between anchor and soil layer. The anchoring effect is added to transmit tensile force of free section to the soil of deep depth.

Advantages of self drilling mining anchors:

  1. Self-contained drill bit, which can be used for self-drilling mining anchors, which have both the functions of drill rod and anchor rod. The process of inserting anchor rod into drill rod is cancelled, which can prevent rework due to hole collapse
  2. Mining bolt body can be extended, with a variety of specifications, easy to use.
  3. Compared with solid rod with the same cross-sectional area, hollow rod has greater resistance, bending resistance and surface adhesion.
  4. Mining anchor is hollow, and can be grouted to ground through mining anchor. General mining anchors are solid steel bars, while self drilling mining anchors are hollow steel pipes. (Mine anchors are grouting pipes. Mortar enters from the end of the mine anchor, passes through the hollow web and hollow drill, and is used from mine. The front end of  anchor rod overflows, and  grout comes back to the end, which can ensure  filling of mine anchor to turn around open ground and consolidate rock mass, fully ensuring grouting and anchoring quality. The stopper plug can ensure dense grouting and can reach a pressure of 1.5 ~ 2.0MPa.)
  5.  Mining anchor is equipped with arched backing plates and nuts, which can apply prestress to the ground.
  6. It can be drilled by ordinary drilling machinery, such as hand-held wind drill.

What Types of Anchor Bolts are used as Wind Tower Fastener

Wind Tower Anchor Bolt

The Importance of Wind Tower Anchor Bolt

Chinese wind power industry have developed from the stage of digesting and introducing overseas technologies to the stage of independent technological innovation, which compared with other countries, has advantages in terms of both technology and price. In recent years, China’s wind power industry, especially large-capacity megawatt-scale large-scale wind power generators, has developed rapidly. Wind power fasteners are the main connector of wind power generators. In power generators, high-strength anchor bolts are used in large numbers. It accounts for about 22% of the total number of wind turbine components. Pitch bearing and blade, hub and pitch bearing, hub and main shaft, main frame and yaw bearing, main frame and generator frame, main bearing and main frame, gear box elastic support and main frame, tower and offset high-strength anchor bolts are used to tighten the brake discs, the connection between tower parts, and locking discs used to connect main shaft to gearbox planet carrier. It can be seen that almost all key components are connected by high-strength anchor bolts. The cost of a wind turbine’s fasteners accounts for about 3.5% to 6.0% of the total equipment cost. Although the parts are small, they have a great effect. High-strength anchor bolt for wind power require strong fan stability due to long-term field service, harsh environments, and poor maintenance conditions. Under normal continuous working conditions, wind power fasteners must guarantee a service life of more than 15 years.

Wind Tower Anchor Bolt Production

Due to randomness of its power source, harshness of operating conditions, particularity of manufacturing and installation, and high cost of maintenance, wind turbines have set extremely high requirements for anchor bolted connections. It is necessary to start from its characteristics in structural design and manufacturing. Necessary measures must also be taken on the process, workshop and on-site production assembly to ensure that anchor bolt connection is foolproof. Most of high-strength anchor bolts for wind power use 10.9 grades, and a few are 8.8 or 12.9 grades. High-strength anchor bolt quality for wind turbines is greatly affected by raw materials. From a series of aspects such as appearance quality, chemical composition, low magnification structure, depth of decarburized layer, non-metallic inclusions, microstructure (grain size) and upset test, all have a significant impact on anchor cage foundation bolt quality . For high strength anchor bolt fastener industry, wind power fasteners will undoubtedly be a huge market. Opportunities and risks coexist in the wind power fastener market.

Classification of Wind Tower Anchor Bolt

At present, there are roughly 3 types of fasteners used in domestic wind turbines and foundations:

  1. Tower bolts. High strength bolts used on the wind turbine tower are mainly GB / T 1228 ~ 1231, DIN6914 ~ 6916 and DAST.
  2. Bolts of wind turbine. Or foundation bolt, onshore anchor cage bolt, high strength anchor bolts used on wind turbines are mainly hexagonal head bolts such as GB / T 5782, GB / T 5783, GB / T 70.1, GB / T 6170, and GB / T 97, hexagon bolts, hexagon nuts and washers.
  3. Blade bolts. That is, anchor bolts connecting wind turbine blades are mainly non-standard double-headed studs and T-round nuts, blade bolts, T-nuts and morse washers, etc.

Steel Material Used for Wind Tower Bolts

Anchor Bolt Round Bar Material

High-strength anchor bolt for wind power are widely used in medium-carbon steel and medium-carbon alloy steel with a carbon content of 0.25% to 0.55%. For products ≤ M24, the material is 20MnTiB steel; for M27 and M30 bolts, 35VB steel is used; a small number of products are also allowed to use 30 CrMnSiA steel; ≥ M 3 3 are all 42CrMoA, B7, 40CrNiMoA steel.

Generally, 45 and 35 steels are used for wind power nuts, and some products are specified with 35CrMoA steel; gasket material is 45 steel. The elemental composition of the materials used for bolts, screws, studs, nuts, and washers is directly related to the mechanical properties of the fasteners, so it must not be lower than the requirements of the recommended materials.

Why Choose ASTM A193 B7 Round Billet for Anchor Bolt

astm a193 grade b7 for anchor bolt

Chemical Composition of A193 Grade B7

B7 steel is a high-strength fastener suitable for high-temperature and high-pressure environments in American ASTM A193 / A193M “alloy steel and stainless anchor bolt materials at high temperatures” standard. This special fastener standard specifies chemical composition range of B7 grade,inspection is based on the furnace batch number, so it is suitable for high temperature and low temperature requirements and special structure types. This type of fasteners is mainly used in manufacturing industrial equipment, pipelines and wind turbines.

ASTM A193 Grade B7 Chemical Composition
CarbonManganese, maxPhosphorus, maxSulfur, maxSiliconChromiumMolybdenum
0.08-0.151.000.0400.0301.00 max11.5 – 13.50.15-0.25

ASME / ASTM, ISO898.1 and GB / T3098.1 fastener standards generally do not specifically specify chemical composition of each grade, but only give a wide range, allowing manufacturers to ensure that performance meets standard requirements. According to respective process conditions, it is selected from many blanks; delivery inspection of finished product is carried out according to production batch, and larger batch can be composed of different brands and different batches, so quality level is equivalent to (slightly higher) than ordinary standard parts.

Why Choose B7 Grade For Anchor Bolt Production

Astm a193 Grade B7 Round Billet

B7 steel has a wide range of chemical composition, especially carbon content. In the U.S. alloy structural steel system, B7 includes totally 6 grades such as AISI 4140, 4142, 4145, 4140H, 4142H, 4145H steel, compared with the 35CrMoA anchor bolt material grade commonly used in GB / T307721999 “alloy structural steel”, not only significantly increases carbon content, Reached the level of 42CrMoA material; and Mn content is also high.

Therefore, fastener companies must pay attention to this difference when preparing round billets. The billets of 2 types of materials cannot be used interchangeably.

Excellent Mechanical Properties of Grade B7 Steel

B7 is a high hardenability steel for quenching and tempering. Its mechanical properties, composition (especially carbon content of elemental that affects hardenability), heat treatment process and cross-sectional size are closely related. According to hardenability test, critical diameter of B7 oil quenching is 42-45 mm. Therefore, although composition range of B7 is relatively wide, in fact, it is to meet the needs of varies product specifications, and at the same time, it gives the fastener manufacturer a greater choice of round billets and heat treatment process systems.

B7 steel has a wide applicable temperature range, up to 538 ℃. After tempering, it has good low temperature toughness. Generally, brittle transition temperature is below (-50 ℃). The ASME pressure vessel code stipulates minimum temperature for B7 steel to be exempt from impact energy test is-48 ℃.

When working temperature of fastener is lower than -48 ℃, a low temperature impact energy test must be performed on B7 steel. At this time, corresponding AISI 4040H, 4142H, and 4145H steels should be selected to ensure low temperature impact performance. The chemical composition of this type of steel is basically the same of B7 steel. In addition to steel strength, main quality indicators must also include low temperature impact work tests.

Analysation of Self Drilling Rock Bolt Advantage in Application Fields

Self Drill Rock Bolt Drilling

First you need to confirm drilling depth, and it shall not be less than effective length of rock bolt bar, extra length of rock bolt is usually less than 100mm. Drilling direction should be perpendicular to excavation surface. If excavation surface is not parallel to anchor surface, vertical deviation from excavation surface should not be greater than 20°; self drilling rock bolt arrangement is relatively concentrated, so waterproof work of external under basement floor is more convenient.

The drilling depth of self drilling rock bolt should not be less than the effective length of the anchor rod body, overlength of rod body should not be greater than 100mm. The drilling should be straight, and drilling direction of the system should be perpendicular to the excavation surface.

Self Drilling Rock Bolt

Advantages and Disadvantages of Self Drilling Rock Bolt

For the case where self-drilling rock bolt have insufficient bearing capacity, due to the fact that with more self-propelled anchors sharing, the stronger resistance forms. Let’s look at the advantages and disadvantages of 3 self-propelled anchor arrangements.

  1. Self drilling rock bolts installed in points around columns; Advantages: the vertical force transmitted from upper structure can be fully used to balance out some of water buoyancy; due to the concentrated arrangement of rock bolts, basement floor waterproof construction is also more easy, there is more resistance due to more anchor rod sharing in case bearing capacity of rock bolt at certain point is not enough.
  2. Self-drilling rock bolts are installed in concentrated line, The vertical force transmitted by the superstructure cannot be fully used to balance out some of the water buoyancy (personally, it is considered unsafe if considered, for floor beams with a span-to-height ratio less than 6, the vertical force transmitted by the superstructure can be properly considered To balance out some of the water buoyancy), it is required to be anchored in hard rock, not suitable for soft rock and soil; the basement floor plate is relatively reinforced.
  3. Self-drilling rock bolts are uniformly arranged in the surface, evenly under the basement floor; advantages: applicable to all soil and rock masses; basement floor beams and slabs have less reinforcement. Disadvantages: The vertical force from upper structure cannot be fully used to balance out some of water buoyancy self-drilling rock bolt.

Anchor Bolt Important Role in Wind Turbine Foundation

Wind Turbine Foundation Design

Anchor Bolt for wind turbine cage foundation is used to attach wind turbine to steel rebar and concrete structure. Anchor cage bolt requires high strength as well as anti-corrosion characteristic, so anchor bolt manufacturing include 2 processes: anchor bolt production and anti corrosion treatment. Anchor Bolt is mostly made of high strength steel 42Crmo steel rod, it is fastened onto anchor ring plate to form a cage shape and fixed with steel rebar and concrete structure for onshore wind turbine foundation. .

Why Anchor Bolt is So Important

Wind turbine anchor cage foundation failure always related to 2 parts: anchor bolt strength failure and concrete structure crack failure. Anchor bolt is the connection between anchor ring plate and steel concrete structure, if anchor bolt fails, it will lose the integrity and lead to further failures. Anchor cage foundation design, bolt length, configuration, materials are also critical to project safety.

Anchor Cage Bolt Length

Anchor cage bolt for wind turbine is a cast-in-place high strength fastener, wind turbine is a heavy equipment which need to be mounted on poured concrete structure. Why anchor cage bolt is long? Accident record of anchor cage bolt shows that short anchor bolt has higher risk of anchor bolt failures of crack in horizontal direction,as well as adding post-tensioning effect. A qualified designer can give a cost efficiency plan according to reality.

Anchor Cage Design for wind Tower Foundation

Main product of LongTeng is Anchor Cage foundation for wind power tower of onshore projects. In all, Longteng purchase high strength steel rod for anchor bolt assembly production, anchor cage will not be installed before delivery, installation is accomplished on site by professional workers.

Rock Bolt Rod Material Heat Treatment

High Strength Steel Rebar

Rock bolt rods are usually made of high-quality alloy steel. The most commonly used materials are medium-carbon alloy structural steels such as 40Cr , 35CrMo and 42CrMo with good hardenability. To prevent corrosion, rock bolt body should be blue or blackened.

Technical Requirements for Heat Treatment of Rock Bolts

40Cr , 35CrMo , 42CrMo and other materials after quenching, hardness ≥ 50HRC , 95% of the core is medium-sized martensite structure; after tempering is 32 ~ 38HRC , tensile strength is 95kgf / mm2 (1kgf / mm2 = 9. 8MPa) , yield strength ≥ 80kgf / mm2 , elongation ≥ 10% , reduction of area ≥ 42% , impact toughness ≥ 6 ~ 7kgf / mm2 . The heart is a uniform tempered sorbite, allowing a small amount of free ferrite (1 to 3 qualified).

In order to ensure heat treatment quality of rock bolts, rock bolts after hot-punching hexagon should be spheroidized and annealed before final quenching. The annealing process specification is: 680 °C × 3h, then temperature is raised to 770 °C × 2h , and furnace is cooled to 550 ~ 580 °C × 2h, it is air-cooled and has a hardness of HB170 to 207 to obtain a spherical pearlite structure. Then prepare for subsequent quenching and tempering treatment.

Heat Treatment Process Details of Rock Bolts

In order to meet the above heat treatment technical requirements of rock bolt, heat treatment process should be in a salt bath furnace or in a continuous controlled atmosphere furnace (mesh belt), and heating temperature is selected from a high quenching temperature of 860 to 870 °C. This process is to make spherical pearlite completely transform into austenite structure, and fully exert beneficial effect of carbon and chromium to increase hardening. Holding time in a salt bath of 15 ~ 20min , then quench 20 is ~ 60 deg.] C to 10 ~ 30 for No. machine oil, quenching hardness should be ≥ 50HRC . The tempering is water-cooled in a pit furnace or a salt bath at 520 to 550 °C for 90 minutes , which can effectively avoid the occurrence of secondary temper brittleness, obtain tempered sorbite structure, and the hardness after quenching and tempering treatment is in HRC30. Within range of ~ 38 , the requirements for mechanical properties are ensured. If continuous mesh belt furnace is used, computer controls atmosphere, temperature and process parameters. With highly mechanized and automated equipment, and final product quality is stable. It is preferred heat treatment equipment for standard parts industry at home and abroad.

Threaded Rock Bolt Samples
Rock Bolt Plate

It should be noted that in order to avoid obtaining a coarse quenched martensite structure, quenching temperature should not exceed 870 °C, otherwise brittleness will increase, and even after high temperature tempering, it will cause fatal defects of fracture during installation operation. Quenching cooling medium uses oil instead of water, which is considered to prevent quenching of hexagonal head while still being hardened. Surface decarburization is one reason for short service life of connecting rock bolt. Therefore, atmosphere or deoxidation in heating furnace should be strictly controlled.

During quenching and tempering treatment of connecting rod bolt, defects of metallographic structure mainly include surface decarburization, excessive free ferrite or excessive undissolved carbide and overheating. These defects will affect rock bolt performance.

Anti-corrosion Methods of Rock Bolt

Fully Threaded Rock Bolt

The hardness of surrounding rock in the roadway during rock bolt support affects supporting performance. If surrounding rock is very soft, it will be deformed easily, which will increase difficulty of roadway maintenance. If rock hardness is very high, surrounding rock has a strong load capacity, and its stability will naturally increase, so it is easy to maintain. The hardness of surrounding rock determines its load capacity, if capacity is strong and hardness is sufficient, the maintenance effect naturally becomes better. At the same time, anti-corrosion of rock bolt is very important. which need to be paid with close attension.

Anticorrosion Method for Rock Bolt Anchorage Section

  • In a typical permanent corrosive environment – rock bolt, bolt anchor rod segment can be employed within mortar or grout closed corrosion, and it must be surounded with protective layer 20mm thick.
  • In severe corrosive environments permanent – rock bolt, bolt rod inside the anchoring section is good bellows jacket, cement slurry or voids in epoxy resin filling and thickness of protective layer surrounding the sleeve not less than 10mm.
  • Temporary bolt anchorage section should be closed with cement slurry to prevent corrosion, and protective layer thickness around the pole should not be less than 10mm.

Anti-corrosion Method for Exposed Anchor Portion

Self Drilling Mining Rock Bolt

When permanent-long-lasting rock bolt adopts the exposed head, it must be coated with anti-corrosion materials such as asphalt, and then sealed with concrete. Protective layer thickness of the exposed steel plate and anchor should not be less than 25mm.

Anticorrosion Method for Free Section of Rock Bolt

The rock rod surface in free section of permanent-long-lasting anchor is coated with anti-corrosive paint or lubricating oil, and then wrapped with plastic cloth, coated with a layer of lubricating oil on the plastic cloth surface, and then loaded into a plastic sleeve to form a double-layer anti-corrosion.

The temporary anchor free section rod body can be coated with anti-corrosive paint or lubricating oil, and then wrapped with plastic cloth for easy anti-corrosion measures.

Heat treatment Technology in High Strength Anchor Bolt Production

Anchor Bolt Heat treatment

Ultimate Performance of Wind Turbine Anchor Bolt

Accelerating the development of heat treatment has become China’s national strategy. In the context of major changes of international environment, starting from current situation and existing problems of heat treatment in China, necessity, urgency and importance of accelerating the development of anchor bolt heat treatment have been highlighted in front of us.

“Heat treatment” is a technique that uses heating and cooling methods to control phase transitions, microstructures, residual stress fields, and to impart extreme properties to materials and high strength bolts for serviceability. It is both a theoretically strong science and a very practical type of technology. The practicality is that advanced heat treatment process is ever-changing with materials and high-strength wind anchor bolts, and it is constantly innovating, continuously developing. It can be seen that no material can leave the heat treatment, and no high strength bolts do not rely on heat treatment. It is important to point out that heat treatment is one of the only two techniques that today gives the ultimate performance of materials of high-strength bolts.

Heat treatment differs from blanking technology and cutting technology. Cold-formed blanks, forged blanks, cast blanks and welded blanks can produce blanks of a certain shape and size, which are conspicuous. The cutting process can produce a variety of precise and beautiful high-precision bolts, which are extremely eye-opening. The heat treatment technology is “internal medicine” of wind foundation anchor bolt manufacturing.

Development of High-Strength Anchor Bolts - Heat-treatment

Advanced manufacturing technology is the technical support for development of advanced high-strength bolts. Heat treatment technology is an important part of advanced manufacturing technology and plays an important role in high-strength anchor bolts and high-end mechanical equipment. Morden Steel mills often recognize the importance of “ingredients” and important role and status of heat treatment. Using highs strength roud bar as raw material, usualy the material is 42crmo, we can get round bar of high strength and better mechanical property with proper heat treatment technology, which Longteng has been used to produce wind turbine foundation bolt for decades, heat treatment is the key to success.

Heat treatment is directly related to improving material power, and it can’t be done without heat treatment innovation. The heat treatment technology in China has accelerated the development of material power, so is the heat treatment technology. It is not known that the lack of attention to heat treatment, lack of innovation and control of the innovative development of advanced materials, no heat treatment innovation, there is no advanced materials.

Facing “Industry 4.0” and “Third Industrial Revolution”, intelligent, green, lightweight and heat treatment technologies play an important role in the development of high-strength bolt manufacturing. For development, we should choose the process that plays a fundamental and breakthrough role in the development of the fastener industry as the key to accelerate development, and attach importance to integrated innovation is the key to improve the core competence of fastener enterprises. The development of heat treatment will be more personalized.

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