<소재> PWHT

PWHT 용접후 열처리
: 용접을 하면 용접된 자리 보다는 주변 금속이 용접 열에 의해 구조적으로 약해진다. 용접 열에 의해 약해진 부위를 HAZ (Heat Affected Zone)이라 하며 약해진 부위에 대하여 잔류응력의 제거, 응력집중 완화 (Stress Relief), 금속조직의 안전화를 위하여 #PWHT 처리가 필요하다. 온도를 올림에 따라 용접부 내의 수소를 제거할 수 있다.

Overview of Post Weld Heat Treatment (PWHT)
 Contribute to Definition
Welding is an essential part of operating and maintaining assets in the petroleum (upstream, midstream, downstream) and chemical processing industries. While it has many useful applications, the welding process can inadvertently weaken equipment by imparting residual stresses into a material, leading to reduced material properties.
In order to ensure the material strength of a part is retained after welding, a process known as Post Weld Heat Treatment (PWHT) is regularly performed. PWHT can be used to reduce residual stresses, as a method of hardness control, or even to enhance material strength.

*PWHT방법
#써모커플, #thermocouple 을 사용하여 가열부 온도를 계측하면서  온도를 올린다.
노내, 국부가열 방법이 있다. 25mm당 1시간정도.. BS, JIS, ASME 기준이 있음
API 5L XI65에서 온도구간 확인 필요 (재질에 따라..)
or ASME Section III 코드
*Piping
- ASME/ANSI B31 Code
*Storage Tank
- API 650(상압저장조,Atmospheric Storage Tank-AST),620(Low Pressure Storage),653(AST를 보수등을 할경우)
*Boiler and Pressure Vessel
-ASME BPVC Section VIII

그래프 읽는 법
시작점 : 기준온도 : 후열처리 시작온도
가열 : 가열속도도 중요함 (파랑색선)
홀딩타임 : 최소유지 온도 및 시간 (빨간선)
냉각 : 냉각시간 (초록색선)

예시: 홀딩온도 700도씨, 두께 1000mm, 700도씨*(25/1000) = 17.5도씨/시간

If PWHT is performed incorrectly, or neglected altogether, residual stresses can combine with load stresses to exceed a material’s design limitations. This can lead to weld failures, higher cracking potential, and increased susceptibility to brittle fracture.
PWHT encompasses many different types of potential treatments; two of the most common types are post heating and stress relieving:
Post Heating:
Hydrogen induced cracking (HIC) often occurs when high levels of ambient hydrogen permeate into a material during welding. By heating the material after welding, it is possible to diffuse hydrogen from the welded area, thus preventing HIC. This process is known as post heating and should begin immediately after the weld is completed. Rather than being allowed to cool, the material needs to be heated to a certain temperature depending on the type and thickness of the material. It should be held at this temperature for a number of hours dependent on the thickness of the material.
Stress Relieving:
By the time it’s complete, the welding process can leave a large number of residual stresses in a material, which can lead to an increased potential for stress corrosion and hydrogen induced cracking. PWHT can be used to release these residual stresses and reduce this potential. This process involves heating the material to a specific temperature and then gradually cooling it.
Whether or not a material should undergo PWHT depends on a number of factors, including things like its alloying system or whether it’s been subject to heat treatment previously. Certain materials can actually be damaged by PWHT, while others almost always require it.