Failure Analysis investigation of cracked studded pipe used for Convection Module for heat recovery in oil and gas refineries application.

Failure Analysis finding:

High temperature studded piping having two sections of

316H austenitic stainless steel were found to be leaking in one

location each during hydrotesting. A sample, including one of

the leaking areas, was cut and investigated for a failure reason

to check the leaking issue. Following preliminary examination

and Dye Penetrant Inspection (PT), the sample was further

investigated for metallurgical investigation and SEM/EDS

examination.

A general view of the sample having crack is shown in figure 1.

As visible below, the failed area was marked in yellow, and by

means of a white band taken for failure analysis investigation.

Technical Background

The leaking pipe section is part of a convection module for heat

recovery. The pipe is made from grade AISI 316H to which some

carbon steel studs were joined by high-frequency resistance

welding. Before commissioning, the system was subjected to hydrotesting, and two leaks were found in the tube under

investigation.

Preliminary Visual Examination and Dye Penetrant

Inspection

Before any further sampling operation, the internal surface

of the pipe was cleaned and then subjected to dye penetrant

inspection (PT). PT confirmed the presence of only one

indication open on the pipe surface as marked by a black box in

figure 2. As better visible in figure 3, the leaking area is affected

by a linear indication consistent with a longitudinal crack.

Furthermore, the pipe appears affected by localized bulging

along the circumferential areas marked by yellow arrows as

illustrated in in figure 2.

Optical Emission Spectroscopy (OES) was carried out on a sample cut from the pipe section. The results were then compared with

the nominal chemical composition for grade AISI 316H. As presented in table 1, the analysis results are consistent with the nominal

composition for the grade under investigation.

Visual and Stereo-Microscopy

Examination

A general view of both sides of the

sample, selected for further investigation,

is shown in figure 4. In detail, the crack

location is marked in figure 4a and 4b

for the external and internal surface,

respectively. As evident in figure 4b, the pipe surface exhibits heat tints.

A general view of the crack path, as

better visible by stereomicroscopy, is

shown in figure 5 and 6 close-up view

of the crack. As shown in the close-ups

below, the internal surface exhibits an

intergranular pattern.

Micro sectioning was carried out as

illustrated in figure 7. As visible below,

the right portion of the crack was

forced open to allow stereo and SEM

fractography (fig. 8). The remaining part

of the crack was used for metallographic

examination along the transverse

direction. Two views of the crack

propagation path, with respect to the

stud positioning and welded joint was investigated for crack propagation

pattern to study.

Metallographic examination

The representative micro-section of the

cracked area, taken transverse to the leak

as illustrated in figure 7, was subjected

to metallographic preparation and

examination prior and post etching.

A general view of the selected area is

visible in figure 8 where the regions subjected to observation are illustrated. As evident below,

the main crack path runs transversally through the pipe wall

thickness with one end located at the lack of fusion between the

right stud and the pipe external surface (figures 9 and 10b).

Additional micrographs of the cracked area are verified prior

and post etching respectively. Based on the metallographic

evidence, the crack path appears to be predominantly

intergranular in nature. Presence of a copperish-coloured

matter was also observed along the crack internal surfaces.

The sample exhibits a microstructure considered consistent

with the condition of the austenitic grade under examination.

SEM examination and EDS Analysis

Following cleaning in ultrasonic bath, Scanning Electron

Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

were performed in the most relevant and representative

regions of fracture surface in order to investigate the mode of

fracture.

Based on SEM fractography, the crack propagation path,

through the pipe material, was confirmed to be intergranular

nature as evident in figures 11-12 at increasing magnification,

this confirming a brittle mode of failure.

Evidence of additional features, considered not genuine of

the base metal intergranular pattern, was observed and better

investigated by EDS elemental analysis as illustrated in figures

12. Elemental analysis confirmed the additional features to be

mainly composed of copper. Copper was also detected during the examination and analysis of the crack internal surface (in

cross- section) as shown by elemental mapping in figures 13-14.

An additional microsection was cut through a region far from

the leaking one as shown in figure 15. Following metallographic

preparation, a crack was observed in area B as illustrated below.

Area B and A, in the HAZ, were selected for EDS analysis.

In detail, two regions of the crack in area B were analysed as shown in figures 16-19. Based on EDS results, the first region

analysed showed presence of oxygen and carbon inside the

crack whereas only carbon was fond in the second region.

The HAZ area denominated as A, and subjected to EDS

analysis, is shown figures 20-21. As presented by elemental

mapping, no evidence of contamination but the silicon coming,

likely, from the preparation consumables, was detected.

No evidence of copper was found in these additional three

areas selected for further investigation.

Discussion and conclusions

Based on the results of the present investigation, the root cause

of the cracking issue affecting the pipe sample submitted for

investigation is considered to be copper contamination of the

austenitic stainless lattice.

Metallographic and SEM examination revealed the cracking

issue to have propagated along a path intergranular in nature.

Presence of copper was also observed at first by optical

microscopy and then was confirmed by EDS analysis, on both

the opened fracture surface and on the internal surface of the

crack.

The presence of copper along the crack path, in addition

to an intergranular mode of fracture for the grade under

investigation, is considered consistent with a liquid metal

embrittlement of austenitic stainless steels by copper.

Such a phenomenon is known to occur following to copper

contamination provided that the material is also subjected to

high temperature exposure.

Although a deeper study into the actual mode of contamination

is still required, the above-mentioned scenario is considered

consistent with the pipe manufacturing process related to the studs welding stage. Reportedly, the welding operation

involves tools with parts made from copper. Furthermore, the

welding process is believed to have provided an increase in the

temperature sufficient for this detrimental mechanism to occur.

Local overheating of the pipe during welding, given the heat tint

and bulging observed on the sample cannot be excluded.

The pipe failure must be ascribed to an intergranular

decohesion mechanism that led to the formation of the

intergranular cracks observed by SEM and eventually the

brittle fracture through the pipe wall. In summary:

• The chemical composition of the pipe sample is considered

consistent with the nominal composition stated by ASM

Metal handbook for grade AISI 316H.

• Dye penetrant inspection revealed the presence of a linear

indication consistent with a crack along the longitudinal

direction of the pipe.

• Stereo and metallographic microscopy observations

revealed the main crack path to be predominantly

intergranular in nature.

• SEM fractography confirmed the intergranular nature of

the crack and a brittle mode of failure for the austenitic

lattice.

• EDS analysis confirmed the presence of copper along

the crack internal surface both on a microsection cut

transverse to the main crack and on the fracture surface

which became accessible following to crack opening.

• The observed intergranular brittle failure of the pipe is

considered consistent with a liquid embrittlement scenario

occurred in presence of copper and post exposure to high

temperature.

• A study into the origin of the copper contamination is

strongly recommended.