This standard specifies the inspection contents, methods and treatment measures of high-temperature bolt of fossil-fired power plant and proposes requirements for the firmness and disassembly of bolt.
This standard is applicable to the inspection and treatment of steam cylinder and valve (working temperature over 400℃), various valves as well as the bolt, nut and gasket of steam pile line flange, This standard may be taken a reference for the fasteners with working temperature lower than 400℃.
2 Normative Reference
The following documents contain provisions which, through reference in this text, constitute provisions of this standard. For dated reference, subsequent amendments to (excluding corrigenda), or revisions of, any of these publications do not apply. However, all parties coming to an agreement according to this standard are encouraged to study whether the latest edition of these documents is applicable. For undated references, the latest edition of the normative document is applicable to this standard.
GB/T 2 Fasteners-Ends of parts with external thread
GB/T 3 Run-outs, undercuts and chamfers for general purpose metric screw threads
GB/T 90.1 Fasteners--Acceptance inspection
GB/T 90.2 Fasteners--Marking and packaging
GB/T 98 Specification for tab washer
GB/T 196 General purpose metric screw threads - Basic dimensions
GB/T 197 General purpose metric screw threads—Tolerances
GB/T 231.1 Metallic materials--Brinell hardness test--Part 1: Test method
GB/T 5779.1 Fasteners-Surface discontinuities-Bolts, screws and studs for general requirements
GB/T 5779.2 Fasters - surface discontinuities - nuts
GB/T 6394 Metal-methods for estimating the average grain size
GB/T 17394 Metallic materials--Leeb hardness test
DL/T 441 Supervision code for creep strain of steam pipe in high temperature and high pressure in fossil fuel power plant
DL/T 694 The technical guide of ultrasonic inspection for high-temperature tight bolts
DL/T 884 Power plant metallography inspection and assessment guideline
JB 4730 Nondestructive testing of pressure vessels
3 Material Selection Principle
3.1 Good anti-relaxation property enables the residual preload stress of bolt after running a design cycle under lower preload stress is still higher than the minimum sealing stress.
3.2 The strength coordinate with plasticity well and the creep notch sensitivity is small. The permanent plasticity of smooth specimen when the bolt material over 8000h~10000h is generally required to be: new material is larger than 50%; the running materials shall not be less than 3%.
3.3 The structure is stable and trend of hot brittleness is small.
3.4 Good oxidation resistance to prevent the seizure of screw bolt and nut due to oxidization of screw thread after longtime running.
3.5 The nut strength should be one grade lower than bolt materials and the hardness should be 20HBW~50HBW lower. The common bolt materials for each working temperature grade are listed in Table 1. If the bolt materials in table are used for nut, the temperature may be 30℃~50℃ higher than that list in Table1. See Annex A for the chemical composition and mechanical property of common bolt materials; see Annex B for the heat treatment process.
3.6 The fasteners for the same flange shall be adopted with same steel type, strength grade and structure form in principle. If fasteners of different materials and strength grades shall be installed on the same flange, the influence caused by different linear expansion coefficient and anti-relaxation property may be calculated.
