Boiler Terminology Explanation (Part 11)

Boiler Terminology Explanation (Part 11)

101、Brittleness of Metal
The characteristic of metallic materials absorbing minimal mechanical energy during fracture, manifested as failure without macroscopic plastic deformation. The occurrence of brittle or ductile fractures in metals depends not only on material properties but also on environmental conditions (e.g., temperature, medium), component geometry (particularly critical for precision components like the B381 F2 gate valve), surface state, stress conditions, and loading rate. Brittleness is typically characterized by impact values and their variations. Based on triggering conditions, it is categorized into red brittleness, cold brittleness, temper brittleness, hot brittleness, and aging brittleness.

102、Red Brittleness
A brittleness exhibited by metals at 800–900°C or higher, also known as hot shortness. It commonly occurs in sulfur-rich or poorly reduced steels, leading to cracking during high-temperature forging. The primary cause is sulfur existing as iron sulfides or oxides, forming low-melting-point eutectics distributed along grain boundaries. At temperatures above 800°C, these eutectics melt, weakening grain boundary strength and inducing brittle fracture.

103、Cold Brittleness
Brittleness displayed by metals at low temperatures. This phenomenon occurs only in body-centered cubic lattice metals (e.g., iron). Carbon steels and low-alloy steels used in boiler manufacturing are susceptible. To prevent cold brittle fractures, brittle-to-ductile transition temperatures are determined via impact or drop-weight tests. Materials with transition temperatures below operational temperatures must be selected.

104、Temper Brittleness
The embrittlement of certain quenched alloy steels after tempering in specific temperature ranges. It is classified into two types:

Type I (250–400°C): Irreversible temper brittleness, predominantly in alloy structural steels, causing intergranular fracture.

Type II (500–550°C): Reversible brittleness in chromium, manganese, and nickel-chromium steels. Additions of molybdenum/tungsten or rapid cooling after tempering can mitigate Type II brittleness. Reheating to 600°C followed by fast cooling also eliminates it.

105、Hot Brittleness
A phenomenon where steels held at 400–550°C for extended periods exhibit significantly reduced impact values after cooling. Common in low-alloy Cr-Ni steels, Mn steels, and Cu-containing steels (Cu ≥0.04%). Hot brittleness is attributed to grain boundary precipitation of brittle elements (e.g., phosphorus, carbides, nitrides), such as in thermal power plant high-temperature bolts.

106、Aging Brittleness
Reduction in impact values of cold-worked steels after prolonged exposure at room temperature or 100–300°C. Aging sensitivity is quantified by comparing impact values of pre-strained (10%) specimens (aged at 250°C for 1h) with original material.

107、Ductile-Brittle Transition Temperature (DBTT)
The temperature range where metals transition from ductile to brittle fracture behavior, also termed Fracture Appearance Transition Temperature (FATT). Above DBTT, fractures are ductile; below DBTT, fractures are brittle. FATT is determined by the temperature at which the fibrous-to-crystalline fracture area ratio meets specified criteria.

108、Metal Hardness
A measure of a metal’s resistance to deformation. Higher hardness correlates with greater strength and wear resistance but reduced ductility. Common testing methods:
Indentation hardness (resistance to plastic deformation)
Dynamic hardness (deformation energy)
Scratch hardness (wear resistance)
Factors include composition, microstructure, processing history, and temperature. Hardness testing is non-destructive and widely applied.

109、Fatigue
Progressive crack initiation and propagation in materials/structures under cyclic loading, ultimately leading to failure. Fatigue fractures occur at stresses far below tensile strength and lack macroscopic plasticity, posing catastrophic risks.

110、Creep
Time-dependent plastic deformation of metals under sustained stress. For power plant components (e.g., steam pipes, turbine shafts), significant creep occurs above 0.4Tm (Tm = melting point). Low-melting-point metals (e.g., lead, tin) exhibit creep even at room temperature.