API571-Part4

4.2.8 Creep & Stress Rupture 蠕变和应力断裂非API 510/570考试科目

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Creep & Stress Rupture 700oF ~ 1000oF

























































800oF for C Steel

Plain carbon steel

875oF for for C ½ Mo Stee Steell

C- ½ Mo

Spheroidization

850oF ~ 1400oF

Low alloy steel up to 9% Cr

Tempered Embrittlement

650oF~ 1070oF

2 ¼ Cr-1Mo low alloy steel, steel, 3Cr-1Mo 3Cr-1Mo (lesser (lesser

Graphitisation

extent), & HSLA Cr-Mo-V rotor steels Strain Aging

Intermediate

Pre-1980’s Pre-1980’s C-steels C-steels with large large grain grain size and and C- ½

temperature

Mo

885oF embrittlement

600oF~ 1000oF

300, 400 & Duplex SS containing ferrite phases

Sigma-Phase

1000oF~ 1700oF

300, 400 & Duplex SS containing ferrite phases

Below DTBTT

C, C- ½ Mo, Mo, 400 400 SS

Embrittlement Brittle Fracture





























4.2.8 Creep and Stress Rupture 蠕变和应力开裂 4.2.8.1 Description of Damage 描述 a) At high temperat temperatures ures,, metal metal component components s can slowly slowly and continu continuously ously deform under load below the yield stress. This time dependent deformation of stressed components is known as creep. b) Deformati Deformation on leads leads to damage damage that that may may eventuall eventually y lead to a rupture. rupture.

在高温和载荷低于屈服应力下,金属部件缓慢, 持续下的变形.这变形导致损伤, 最终可能破裂. 4.2.8.2 Affected Materials 受影响的材料 All metals and alloys.所有的金属和合金。

























































































































































































































































































































































































































































































































































































































































































































































































Figure Figure 4-17 4-17 – Pinch Pinched ed Alloy 800H pigtail opened up creep fissures on the surface.

























































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API510/570-Exam

d) Tubes Tubes in the high high temp tempera eratur ture e supe superh rheat eater er or reheat reheater er that that pene penetra trate te through the cooler water-wall tubes may crack at the header connection if the tube is not sufficiently flexible. These cracks are most common at the end where the expansion of the header relative to the water-wall will be greatest.穿过水冷冷却器壁的过热器,再热器管路. d) Steam actuated actuated soot soot blowers blowers may cause cause thermal thermal fatigue fatigue damage damage if the the first steam exiting the soot blower nozzle contains condensate. Rapid cooling of the tube by the liquid water will promote this form of damage. Similarly, water lancing or water cannon use on water-wall tubes may have the same effect. (1) 蒸汽驱动的吹灰器接触冷凝液的热管路, (2) 其他接触冷凝液的热管路服务.





























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4.2.9.5 Appearance or Morphology of Damage 破坏外观形貌 a) Thermal Thermal fatigue fatigue cracks cracks usuall usually y initiate initiate on the the surface surface of the the compone component. nt. They are generally wide and often filled with oxides due to elevated temperature temperature exposure. Cracks may occur as single or multiple cracks. 表面发起点, 开裂表面氧化,裂缝可能为单个或多个裂纹延伸. b) Thermal Thermal fatigue fatigue cracks cracks propa propagate gate transve transverse rse to the the stress stress and they they are usually dagger-shaped, dagger-shaped, transgranular, and oxide filled (Figure 4-24 and 425). However, However, cracking may may be axial or circumferential, circumferential, or both, at the same location. 应力方向横向开裂, 穿晶,氧化,刀纹状(?),开裂方向为; 轴向或圆周向或并存.





























API510/570-Exam

c) In steam steam gener generating ating equipment equipment,, cracks cracks usually usually follow the toe toe of the the fillet fillet weld, as the change in section thickness thic kness creates a stress raiser. Cracks often start at the end of an attachment lug and if there is a bending moment as a result of the constraint, they will develop into circumferential cracks into the tube.在蒸汽发生装置, 通常开裂起点在焊道应力集中点,周圆的延伸. d) Water Water in soot soot blowers blowers may may lead to to a crazing crazing pattern pattern.. The predom predominant inant cracks will be circumferential and the minor cracks will be axial. (Figure 426 to 4-27). 水助吹灰器的裂纹一般上体现为纹状.































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4.2.9.6 Prevention / Mitigation 预防/缓解 a) Thermal Thermal fatigue fatigue is is best preve prevented nted through through desig design n and operat operation ion to minimize thermal stresses and thermal cycling. Several methods of prevention apply depending on the application. 1. Designs Designs that incor incorpora porate te reductio reduction n of stress stress concentr concentrator ators, s, blend blend grinding grinding of weld profiles, and smooth transitions should be used. 减少应力集中,焊





























API510/570-Exam

b) Designs Designs should should incorpora incorporate te sufficient sufficient flexibility flexibility to accommo accommodate date differentia differentiall expansion. 设计时考虑具有足够的灵活性 1. In steam steam generating generating equipment equipment,, slip spacers spacers should should slip slip and rigid attachments should be avoided.滑动垫片. 2. Drain lines should should be provide provided d on soot-blo soot-blowers wers to prevent prevent conde condensate nsate in the first portion of the soot blowing cycle.吹灰器安装排水线避免冷凝液聚集





























API510/570-Exam

4.2.9.7 Inspection and Monitoring 检查和监督 a) Since Since crack cracking ing is usual usually ly surfa surface ce connec connected ted,, visual visual examin examinati ation, on, MT and and PT are effective methods of inspection.由于裂缝通常是表面连接 MT/PT是有效的检查方法. b) External SWUT inspection can be used for non-intrusive inspection for internal cracking and where reinforcing pads prevent nozzle examination.





























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4.2.11.4 Affected Units or Equipment All steam-generating units including including fired boilers, waste heat heat exchangers in sulfur plants, hydrogen reformers and FCC units. Failures can occur in superhea superheaters ters and reheate reheaters rs during during start-u start-up p when conden condensate sate blocks blocks steam steam flow. flow.





























4.2.11.5 Appearance or Morphology of Damage a) These These short-ter short-term, m, high-tem high-tempera perature ture failures failures always always show show an open open burst burst with the fracture edges drawn to a near knife-edge (Figure 4-32). b) The microstructure will always show severe elongation of the grain structure due to the the plastic deformation that occurs occurs at the time of failure.

























































4.2.11.6 Prevention / Mitigation a) When a DNB condit condition ion has has develope developed, d, tube rupture rupture will will quickly quickly follow. follow. Proper burner management management should be practiced to minimize flame impingement. b) Proper Proper BFW treatmen treatmentt can help help prevent prevent some some condition conditions s that can can lead to restricted fluid flow. c) Tubes Tubes shoul should d be visua visually lly insp inspect ected ed for for bulgin bulging. g. 4.2.11.7 Inspection and Monitoring Burners should be properly maintained to prevent flame impingement.





























Causes of Caustic Cracking Caustic is a boiler water additive. It is added to preserve the thin film of iron oxide to protect the boiler from corrosion. The following are the causes of concentration of caustics: 1. Small bubbles of steam nucleated at the metal surface in minute concentrations of solids in the boiler water would be deposited on the metal surface. As the solids are formed they are simultaneously removed from the metal surface by water which re-dissolves them. However, when the rate of bubble formation exceeds the rate of dissolution of the solids, concentration of caustics would begin to increase. 2. The deposits of solids shield the metal from the back water. Steam forms under the deposits and escapes leaving behind a caustic residue. 3. Caustic also concentrates by evaporation if a water line exists. Mechanism Concentrated caustic dissolves the protective magnetite oxides























































































蒸汽封学习重点: a) 易感温度: 高温原理当热流量平衡被打破，单个气泡的加入形成蒸汽毛毯，一种被称

API571-Part4

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