Material, Fabrication, and Repair Considerations for Austenitic Alloys Subject to Embrittlement and Cracking in High Temperature 565 °C to 760 °C (1050 °F to 1400 °F) Refinery Services

API TECHNICAL REPORT 942-B FIRST EDITION, MAY 2017

EFFECTIVE DATE: NOVEMBER 1, 2017

Special Notes

API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed.

Neither API nor any of API’s employees, subcontractors, consultants, committees, or other assignees make any warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API nor any of API’s employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights.

API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any authorities having jurisdiction with which this publication may conflict.

Users of this Technical Report should not rely exclusively on the information contained in this document. Sound busi- ness, scientific, engineering, and safety judgment should be used in employing the information contained herein.

All rights reserved. No part of this work may be reproduced, translated, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 1220 L Street, NW, Washington, DC 20005.

Copyright © 2017 American Petroleum Institute

Foreword

Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent.

The verbal forms used to express the provisions in this document are as follows.

Shall: As used in a standard, “shall” denotes a minimum requirement in order to conform to the standard.

Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required in order to conform to the standard.

May: As used in a standard, “may” denotes a course of action permissible within the limits of a standard. Can: As used in a standard, “can” denotes a statement of possibility or capability.

This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 1220 L Street, NW, Washington, DC 20005. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director.

Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published annually by API, 1220 L Street, NW, Washington, DC 20005.

Suggested revisions are invited and should be submitted to the Standards Department, API, 1220 L Street, NW, Washington, DC 20005, standards@api.org.

iii

Contents

Page

1. Technical Approach/Report Organization and Scope 1

2. Acronyms and Abbreviations 1

3. Process Units 2

   1. General 2

   2. Fluid Catalytic Cracking Units (FCCUs) 2

   3. Hydrogen/Syngas Plants 14

   4. Catalytic Reformers 19

   5. Delayed Cokers 21

   6. Hydroprocessing Units 25

4. Damage Mechanisms 28

   1. Metallurgical Embrittlement 28

   2. Sigma Phase Embrittlement 29

   3. Carburization 46

   4. Stress Relaxation Cracking (SRC) 54

   5. Creep 59

   6. Thermal Fatigue 67

   7. Solidification Cracking 70

Bibliography 75

Figures

1. FCCU Simplified Process Flow Diagram 4

2. Replacement of Primary (Outer Ring) and Secondary (Inner Ring) Cyclones 5

3. Example of New Hexagonal Mesh Welded Inside a Regenerator Cyclone 8

4. Large Areas of Internal Hexagonal Mesh and Refractory that Failed in a Brittle Manner 8

5. Example of External Refractory that Failed After a Short Time in Service 9

6. An Example of a Mitered Joint After Removal From Service 12

7. Creep Failure on a FCCU Regenerator Overhead Line 12

8. A Two Pass SAW Weld Was Found with Creep Cracking in the Outside Weld Bead 3

9. Interdendritic Creep Voids and Cracking in a Weld Cross Section 13

10. Hydrogen Reforming Process Flow Diagram 14

11. Reformer Feed Preheat Coil Arrangement in Units with a Preconverter 15

12. Multiple T/C Shields Welded to an Alloy 800H Superheater Tube 17

13. Cross Section of a Crack at the Toe of One of the TC Shields Showing Intergranular SRC 17

14. Tubesheet-to-Inlet Channel Cone that Cracked in Service 18

15. Cracking From the OD Was Intergranular and Was Attributed to SRC 19

16. Catalytic Reformer Simplified Process Flow Diagram 20

17. Continuous Regenerating Catalytic Reformer 20

18. Delayed Coker Simplified Process Flow Diagram 22

19. Stress Rupture of a Coker Heater Tube 23

20. Carburized Tube that Cracked During Pig Decoking 24

21. Carburized Tube Cross Section Showing Variations in Depth of Carburization 25

22. Hydroprocessing Simplified Process Flow Diagrams for Hydrotreating and Hydrocracking 26

23. Hydroprocessing Simplified Process Flow Diagram of a Hydrotreater with a

    Recycle Hydrogen Heater 26

24. Fe-Cr Equilibrium Phase Diagram Showing Sigma at 40 % to 50 % Cr (Top Axis) 33

25. Isothermal Section of Fe-Cr-Ni Phase Diagram at 650 °C (1202 °F) 34

26. Precipitation of Sigma Phase in Different Grades of Austenitic Stainless Steel at 700 °C (1292 °F) . . . 35

    v

    Page

27. Penetrant Test Showing Sigma Phase Embrittlement Cracking in Type 308H SS Butt Weld 36

28. Cross Section View of Sigma Phase in a Type 304H SS FCCU Regenerator Plenum 37

29. Cold-worked Microstructure Containing Higher Amounts of Sigma Phase 38

30. KOH Etch Revealing the Sigma Phase, but Not the Cold Working 39

31. SEM of a Type 304H SS Cyclone After 14 Years of Operation at 716 °C (1321 °F), 5 % Sigma Phase. . . 40

32. Metallograph of Sigma Phase in a 304H SS Cyclone After 14 Years Operation

    at 716 °C (1321 °F), 5 % Sigma Phase 40

33. Bend Test Results of Type 304H SS with 12 % Sigma 41

34. Tensile Test of 304H SS with (a) 12 % Sigma at 21 °C (70 °F) Depicting Brittle Fracture and

    (b) Ductile Fracture at 716 °C (1320 °F) 42

35. Impact Properties of 304 Type Stainless Steel with 2 % and 10 % Sigma 4 42

36. Charpy V-notch Impact Test Results at Room Temperature and Service Temperature 43

37. Temperature vs. Charpy V-notch Impact Energy of Type 347 SS Weld Metal 44

38. Temperature vs. Charpy V-notch Impact Energy of Type 347 SS Plate 44

39. Relative Severity of Carburization in the Form of Metal Dusting

    for Type 304 Stainless Steel and Alloy 800 47

40. Cross Section of a Type 304H SS Regenerator Cyclone with a 3 mm (0.12 in.)

    Thick Carburized Layer on the ID Surface 49

41. Microstructure at the Transition from Carburized Layer (Right Side) to the Base Metal of a

    304H SS Regenerator Cyclone 50

42. Cross Section of a Stainless Steel Coker Heater Tube with a Brittle Crack 50

43. Light Micrographs Showing Typical Carburized Structures of Nickel Alloys After

    Testing at 982 °C (1800 °F) for 55 h in 5 % H2, 5 % CO, and 5 % CH4 (Balance Argon) 51

44. SEM View, Cr Dot Map, and Fe Dot Map of Carburized Zone Near ID of 347 SS Heater Tube 51

45. Penetrant Examination Results Showing SRC Cracking 57

46. SRC in an Alloy 800H Furnace Tube 58

47. Stress Rupture Curves for Several Annealed Stainless Steels (Extrapolated Data) 61

48. Creep Rate Curves for Several Annealed Stainless Steels 62

49. Three Stages of Creep Damage 63

50. Neubauer’s Classification of Creep Damage from Observation of Replicas [70] 65

51. Two Types of Creep Test Samples, Tangential and Longitudinal 66

52. DeLong Diagram for Estimating Ferrite Content in Austenitic Stainless Steels 72

53. WRC Diagram Including Solidification Mode Boundaries 73

Tables

1. Process Units, Conditions, and Typical Austenitic Stainless Steel Damage Mechanisms* 3

2. Ferrite and Austenite Formers 28

3. Austenitic Stainless Steel Embrittlement Phases and Stress Relaxation Cracking Susceptibility 30

4. Nickel Based Alloy Embrittlement Phases and Stress Relaxation Cracking Susceptibility 31

5. Typical Compositions (wt %) of Select Alloys Shown in Figure 25 33

6. Average Impact Test Results in Joules (ft-lb) Shown in Figure 36 43

7. Heater Tube Bending Ductile and Tensile Test Results 52

8. Influence of Carbon Content in 25Cr-20Ni on Mechanical Properties [48] 52

9. Names for Stress Relaxation Cracking Mechanisms 54

10. Chemistry Requirements of the 800 Alloys 54

11. Stainless Steel Susceptibility for SRC 56

12. Creep Threshold Temperatures [64, 65] 60

vii

Introduction

The API Committee on Refinery Equipment, Subcommittee of Corrosion and Materials, identified a need to develop a technical report focusing on the materials, fabrication, and repair of austenitic stainless steels and nickel-iron- chromium alloys in high temperature 565 °C to 760 °C (1050 °F to 1400 °F) refinery services. Many of these alloys are subject to embrittlement and cracking after prolonged exposure to these temperatures. Susceptible equipment in the following processing units are addressed:

• fluid catalytic cracking units,

  
  

• hydrogen/syngas plants,

  
  

• catalytic reformers,

  
  

• cokers,

  
  

• hydroprocessing units.

This report summarizes industry experience and recommends methods to improve reliability and process safety, and increases industry awareness to high temperature embrittlement issues.

NOTE Embrittlement can be a serious personnel safety issue if plant personnel are not careful about hand-holds and foot-holds when inspecting embrittled piping and vessel components. There has been at least one case where an inspector was seriously injured when an embrittled support failed.

viii

Material, Fabrication, and Repair Considerations for Austenitic Alloys Subject to Embrittlement and Cracking in High Temperature

565 °C to 760 °C (1050 °F to 1400 °F) Refinery Services

1. Technical Approach/Report Organization and Scope

   As a basis of this report, technical literature, industry experience, and published case studies were reviewed. The review included materials of construction, damage mechanisms, and component-specific fabrication and repair issues.

   
   

   The scope of this report includes the following wrought austenitic alloys: Alloys 800, 800H, 800HT®, and 300 series austenitic stainless steels, and corresponding welding consumables. Limits in chemical composition, microstructural requirements, and heat treating practices that mitigate susceptibility to embrittlement and cracking are identified. Potentially viable upgrades to commonly used alloys are identified where applicable.

   
   

   The remainder of this report is organized as follows.

   
   

   Section 3, Process Units, gives a brief process overview followed by an explanation of the various damage mechanisms found in that unit. Component specific considerations and examples of in-service damage are also included. Inspection recommendations and general repair method considerations are also included.

   
   

   Section 4, Damage Mechanisms, contains detailed discussions of high-temperature damage mechanisms; including fundamental details of the solid state reactions, their rate of reaction, and recommended mitigation measures. Section 4 also incorporates fabrication and repair practices that can be used for cracked or embrittled equipment.

   
   

   NOTE Excluded from the scope of this document are Hydrogen Reformer catalyst tubes, outlet pigtails and outlet headers. With the exception of catalyst tubes, these are covered in TR 942-A, Materials, Fabrication, and Repair Considerations for Hydrogen Reformer Furnace Outlet Pigtails and Manifolds. Also excluded are expansion bellows in elevated temperature service.

   
   

2. Acronyms and Abbreviations

   For the purposes of this document, the following acronyms and abbreviations apply. CCC complete carbon monoxide combustion

   CO carbon monoxide

   
   

   CRCR continuously regenerated catalytic reformer CVN Charpy V-notch

   FCCU fluid catalytic cracking unit FN ferrite number

   GTAW gas tungsten arc welding ID inside diameter

   MTR material test report

   
   

   NDE non-destructive examination

   
   

   NDT non-destructive testing

   
   

   1

   MATERIAL, FABRICATION, AND REPAIR CONSIDERATIONS FOR AUSTENITIC ALLOYS SUBJECT TO EMBRITTLEMENT AND CRACKING IN HIGH TEMPERATURE 565 °C TO 760 °C (1050 °F TO 1400 °F) REFINERY SERVICES 2

   
   

   
   

   PASCC polythionic acid stress corrosion cracking PCC partial CO combustion

   PFZ precipitation free zone

   
   

   PT penetrant testing

   
   

   PWHT post weld heat treatment

   
   

   SAGBO stress assisted grain boundary oxidation SRC stress relaxation cracking

   SS stainless steel

   
   

   T/C thermocouple

   
   

   TTT time temperature transformation

   
   

   UT ultrasonic testing

   
   

   UTSW ultrasonic testing shear wave WRC Welding Research Council

3. Process Units

   1. General

      
      

      Table 1 summarizes common embrittlement mechanisms in each of the listed refinery process units. Implications for specific equipment are discussed in more detail in the section for each respective process unit. Information on damage mechanisms can be found in API 571 and in Section 4 of this document.

      
      

   2. Fluid Catalytic Cracking Units (FCCUs)

3.2.1 Process Description

FCCUs are used to process heavy feedstocks, converting them to gasoline, diesel, and furnace oils. A simplified process flow diagram for the FCCU is shown in Figure 1 [1]. The catalytic reaction occurs mostly inside the riser prior to reaching the reactor at temperatures ranging from approximately 480 °C to 565 °C (900 °F to 1050 °F). In modern FCCUs, the “reactor” functions as a hydrocarbon/catalyst separator. During the process, the catalyst becomes deactivated as it becomes coated with carbon (coke). The catalyst is sent to the regenerator where it is exposed to air, promoting the burn off of coke at approximately 650 °C to 780 °C (1200 °F to 1475 °F).

Inside FCCU reactors and regenerators are cyclones which are used to separate the catalyst from the overhead vapor streams. Most regenerators have multiple sets of primary and secondary cyclones. Primary cyclones direct the vapor flow from inside the reactor or regenerator in a centrifugal pattern, forcing the heavier catalyst particles outward against the inside wall, and allowing the catalyst particles to then fall down into the catalyst bed. The lighter vapor stream exits out the top of the primary and into the secondary cyclone to remove residual catalyst from the vapor stream. Primary and secondary cyclones can be seen in Figure 2 [2].

Units that process heavier feeds are called resid fluid catalytic cracking units (resid FCCUs) and these feeds typically have higher sulfur contents. In addition, resid FCCU feeds typically develop higher carbon residues on the catalyst particles. Carbon residues, as indicated by the Conradson Carbon Residue test of Resid FCCU feeds, are significantly higher with up to 5 to 10 wt.%, compared to <1 wt.% for a typical FCCU feed. These heavier feeds also