Flaw studies on the pipes working at high temperature and pressure.

Demian, Mihai ; Demian, Gabriela ; Grecu, Luminita 等

1. INTRODUCTION

Because of many disturbances that appear while energetic groups work a movement of their availability is produced. Disturbances appear at almost all subassemblies from existing boiler in the energetic groups.

While life service of a product there are three kinds of defections that are produced without human utilization causes:

* Premature flaws because un-due fabrication and insufficient control. This kind of flaws appears after their utilization.

* Accidental flaws which cannot be eliminated even by using other materials or a better maintenance policy.

* Flaws made by wear and because of materials ageing. Frequently flaws which appear at boiler's subassemblies are:

1. longitudinal cracks in the area influenced by the welding thermal field;

2. pipe fracture after an initial deformation due to material ageing (steel creep), thermal overstressing, decrement of the pipe wall because of the exterior surface erosion, exterior or interior surface corrosion;

3. pipe fracture without an initial deformation, because of material fragility;

4. pipe diameter increasing due to the steel creep over admitted limits(2% for pipes and 1% for the rest of the elements);

5. pipe curving because of the overheating, stopped dilatation, abrupt fractures;

6. cracks in the areas with stresses concentration. Pipes must obey some conditions

* steel must be dead melted

* S and P must less than 0,045% for carbon steel high alloyed, respectively 0,040% for steel medium and low alloyed

* Cr, Ni, Cu contained in steels not alloyed with them must be, for each of them, less than 0,3%, and their sum less than 0,7%.

* Local longitudinal extension of fracture (A5) must be 18% for carbon steel low a medium alloyed and more than 16% for high alloyed steel.

* Pipe ovality must be :

Ov = 2 * [[d.sub.e] max--[d.sub.e] min/[d.sub.e] max + [d.sub.e] min]/ 100%. (1)

For the maximum admitted imperfections and discontinuities determination for a certain material we must take into account the influence factors:

* Exploitation temperature

* Kind of flaw and its place in the piece

* Induced stress because of the loads

* Analyzed material properties

* Load size and its distribution Plane flaws permit the calculus of the maximum admitted size using relations from cracks mechanics. Maximum stress to work with is obtained by making the sum of different kinds of stresses: bending stress, tension stresses, and stress provided by its own gravity, overstrain. The relation is:

[[sigma].sub.max] = [[sigma].sub.m] + [[sigma].sub.i] + [[sigma].sub.s] + [[sigma].sub.v] (2)

[[sigma].sub.m]--tension stress,

[[sigma].sub.i]--bending stress,

[[sigma].sub.s]--overstrain defined by the concentration of the jointing stress and flaws evaluated with the formula:

[[sigma].sub.v] = ([K.sub.[sigma]]--1)[[sigma].sub.m] (3)

where [K.sub.[sigma]] is the effective concentration coefficient.

Regarding [[sigma].sub.max] values there are different kinds of procedures like:

1. If [[sigma].sub.max] < [[sigma].sub.v], conditions of loads being elastic it is applied linear elastic crack mechanic, (flaw [K.sub.IC])

2. If [[sigma].sub.c] [less than or equal to] [[sigma].sub.max] < 2[[sigma].sub.c], conditions of loads being lasticplastic it is applied linear elastic-plastic crack mechanic, referential criteria being the critical movement at the opening crack

3. If [[sigma].sub.max] > 2[[sigma].sub.c], conditions of loads are plastic, and in this situation must be determinate plastic deformation from the flaw area.

The research want to find what kind of flaw appeared in the pipes when there are used at the finish of life time working and in the future must be determined the flaws with ultrasonic nondestructive tests. (Safta & Safta, 2001)

2. EXPERIMENTAL RESULTS

Two cross-section samples were taken off for analyses from the damaged pipes. On these samples the following analyses were performed: dimensional identification, macroscopic. (Pop et al., 2001)

2.1 Dimensional identification

Sample no. 1

The pipe which provided sample no. 1 exhibits a 110 mm long indentation. The fracture occurred with a flow of metal giving a plastic fracture appearance. From the dimensional point of view the pipe is 38 mm nominal diameter and goes down to 35.5 mm in the damaged area; the pipe's thickness is 6 mm.

Sample no. 2

The pipe which provided sample no. 2, exhibits a 130 mm long indentation and the pipe was bent in a 90 degrees elbow.

2.2 Spectral identification of the steel brand

The following steel brand was identified: X20CrMoV121

2.3 Hardness test

A portable "TIME HLN-11A" was used. The results are spread in a wide range. The no.1 and no.2 samples show hardness within 152--178 HB. Please check Table 1 and 2 for the hardness individual values.

2.4 Metallographic determination

After surfacing mill and polishing the surfaces were treated with a 20% Ammonium Persulfate solution.

The samples were scanned afterwards, using a MC6 Metallographic Microscope with 100x, 150x and 500x magnifying factor. (Bibu, 2000)

Both samples (fig.1 & fig. 2) show a martensite-feritic structure, characteristic for X20CrMoV121 steel. Globular inclusions were found in the sample's structure and also an overheat characteristic structure in the damaged area.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

3. CONCLUSIONS

For a good maintenance of energetic groups, the number of damages must be very small.

An important influence for the decrease damages has the used material and their properties.

The pipes damage occurred due to the material overheating (work temperature exceeding) and also material strength weakening as a result of the steel creep. In order to avoid these kind of damages in the future:

--More temperature measuring probes are required along the heat exchange surface

--Increase the preventive maintenance frequency on the measuring devices

--Increase the scanning frequency for the thermodynamic parameters (steam and water pressure, steam and water temperature)

4. REFERENCES:

Bibu, M (2000.). Materials science, Ferrous and nonferrous alloys, Metallography, Publisher University of Sibiu ISBN 973-651-024-7

Bibu, M.(2000). Ferrous and nonferrous alloys Metallography, Publisher University of Sibiu, ISBN 973-651-027-1

Bibu, M (2000). Methods and techniques of structural analysis of metallic materials, Publisher University of Sibiu, ISBN 973-651-030-1

Deac, V. (2000). Materials science, Publisher University of Sibiu ISBN 973-651-150-2

Deac, V.& Deac, C. (1999). Metalografie, Publisher University of Sibiu, ISBN 973-651-028-1

Mitelea, I (1999). Material science in machine building, Publisher Welding, Timisoara, ISBN 973-30-23-46-9

Mitelea, I (1998), Selection and Use of Materials Engineering, Publisher Polytechnic Timisoara, ISBN 973-9389-01-5

Pop Viorel, Chicinas Ionel, & Jumatate Nicolae (2001), Physical materials. Experimental Methods, Clujana University Press Publisher, ISBN 973-610-036-7

Safta, Voicu Ionel & Safta, Voicu Ioan (2001). Industrial nondestructive flaw detection, Publisher Welding Timisoara, ISBN 973-99425-6- 3

Trusculescu, Marin & Demian, Mihai (2006). Materials Handbook vol.I--Structural Metallurgy, Publisher Polytechnic--Timisoara ISBN 973-625-356-2, 978-973625-356-0

Tab. 1. Hardness measurement for sample 1

Test: 009
1.04.29 STEEL

O LD HB

1 454 165
2 452 163
3 451 162
Ave 452 163

Tab. 2. Hardness measurement for sample 1

Test: 009
1.04.29 STEEL

O LD HB

1 454 152
2 452 163
3 451 178
Ave 452 163