ANALYSIS OF FAILED FORGING SADDLING MANDRELS AND PROCESS IMPROVEMENTS FOR INCREASED MANDREL LIFESPAN Materials Engineering Department Senior Project Brock Costalupes Advisor: Prof. Blair London Industry Sponsor: Schlosser Forge Co. 6/1/2012 Approval Page Project Title: Analysis of Failed Forging Saddling Mandrels and Process Improvements for Increased Mandrel Lifespan Author: Brock Costalupes Date Submitted: June 1, 2012 CAL POLY STATE UNIVERSITY Materials Engineering Department Since this project is a result of a class assignment, it has been graded and accepted as fulfillment of the course requirements. Acceptance does not imply technical accuracy or reliability. Any use of the information in this report, including numerical data, is done at the risk of the user. These risks may include catastrophic failure of the device or infringement of patent or copyright laws. The students, faculty, and staff of Cal Poly State University, San Luis Obispo cannot be held liable for any misuse of the project. Prof. Blair London ____________________________ Faculty Advisor Signature Prof. Trevor Harding ____________________________ Department Chair Signature I Abstract This project seeks to increase the lifespan of steel mandrels used by Schlosser Forge in the saddling process, which is part of the production of forged rings for the aerospace and power generation industries. All saddle mandrel samples were taken from used 4340 steel mandrels. Hardness profiles were measured across rectangular bars cut from the transverse direction of a completely fractured piece of a 7″ diameter mandrel. The hardness near one outer surface of the tested bar was 36 HRC and gradually decreased to 30.5 HRC at the opposite surface. Macroscopic examination of the primary fracture surface along with light microscope imaging of fracture surface feature details suggests that the failure mechanism is likely related to reversed bending fatigue rather than deterioration of the mechanical properties of the steel due to microstructural changes from in-process overtempering during saddling. Cyclic softening of the quenched and tempered steel caused by the saddling process adversely affects the ability of the steel to withstand fatigue. Different heat treatments were conducted to compare the resulting toughness, measured by Charpy impact test, to that of the current process of quenching and tempering the mandrel to a hardness of 38 – 42 HRC. Each heat treatment involved austenitizing at 1500°F for one hour. By varying the tempering temperature, with a constant two hour tempering time, samples were obtained at 28.5 HRC, 39.8 HRC, and 42.5 HRC. Charpy impact energy decreased significantly as hardness increased. An additional heat treatment was conducted where a 4340 steel sample was austenitized, quenched in a 660°F NaNO 3-based molten salt bath and held for two hours. The result of this heat treatment is a bainitic structure, which may show improved toughness. Keywords: forging, materials engineering, saddling, 4340 steel, fatigue, austempered, ring rolling, bainite, superalloy II Table of Contents Abstract ......................................................................................................................................................... I List of Figures ............................................................................................................................................... III List of Tables ................................................................................................................................................ IV Introduction .................................................................................................................................................. 1 Ring Forging .............................................................................................................................................. 1 Saddling ..................................................................................................................................................... 3 Problem Statement ................................................................................................................................... 3 4340 Steel ................................................................................................................................................. 4 Steel Microstructure ................................................................................................................................. 5 Tempering ................................................................................................................................................. 8 Fatigue Fracture ...................................................................................................................................... 10 Realistic Constraints ................................................................................................................................ 12 Experimental Procedure ............................................................................................................................ 13 Hardness Profiles .................................................................................................................................... 14 Metallography ......................................................................................................................................... 15 Heat Treatment ....................................................................................................................................... 16 Fracture Analysis ..................................................................................................................................... 17 Results ......................................................................................................................................................... 18 Hardness Profiles .................................................................................................................................... 18 Metallography ......................................................................................................................................... 21 Fracture Analysis ..................................................................................................................................... 22 Heat Treatments and Charpy Impact Tests ............................................................................................ 23 Discussion ................................................................................................................................................... 25 Conclusions ................................................................................................................................................. 26 Acknowledgements .................................................................................................................................... 27 References .................................................................................................................................................. 28 III List of Figures Figure 1: Ring forging schematic. ................................................................................................................. 2 Figure 2: Diagram of the saddling process ................................................................................................... 3 Figure 3: Fe – C binary phase diagram. ........................................................................................................ 5 Figure 4: 4340 IT curve. ................................................................................................................................ 6 Figure 5: Diagram of the formation of proeutectoid ferrite. ....................................................................... 7 Figure 6: Upper bainite and lower bainite micrographs. ............................................................................. 7 Figure 7: Martensite micrograph. ................................................................................................................ 8 Figure 8: Tempered martensite micrograph. ............................................................................................... 9 Figure 9: Mechanical Properties versus tempering temperature. .............................................................. 9 Figure 10: Schematic representation of the fatigue crack propagation mechanism. ............................... 11 Figure 11: Image of received mandrel samples. ........................................................................................ 13 Figure 12: Hardness profile sample cutting diagram. ................................................................................ 13 Figure 13: Fractured sample hardness profile cutting diagram ................................................................. 14 Figure 14: Zoning diagram for hardness profiles. ...................................................................................... 15 Figure 15: As-opened secondary crack fracture surfaces. ......................................................................... 17 Figure 16: Mandrel end piece hardness profile. ........................................................................................ 18 Figure 17: Fractured mandrel hardness profile 1. ..................................................................................... 19 Figure 18: Fractured mandrel hardness profile 2.