WWW.ABSTRACT.XLIBX.INFO
FREE ELECTRONIC LIBRARY - Abstract, dissertation, book
 
<< HOME
CONTACTS



Pages:   || 2 |

«Keywords: Heat Treatment Simulation, Vacuum Carburization, Gas Quenching, High Alloy Steel, Residual Stress, Distortion Abstract Aerospace ...»

-- [ Page 1 ] --

Predicting Distortion and Residual Stress in a Vacuum Carburized and Gas

Quenched Steel Coupon

A. Freborg, B. Ferguson and Z. Li

Deformation Control Technology, Cleveland, Ohio USA

Keywords: Heat Treatment Simulation, Vacuum Carburization, Gas Quenching, High Alloy

Steel, Residual Stress, Distortion

Abstract

Aerospace transmission components are typically manufactured from high strength, case

carburized alloy steels such as AMS 6308 (Pyrowear®53). The combination of carburization and quench hardening of these steels produces residual compressive surface stresses and high surface hardness, thus enhancing both surface durability and fatigue resistance. The hard case, coupled with a tough non-carburized core, provides the foundation upon which additional processing can further improve surface fatigue response. An internal state variable (ISV) material model for carburized and heat treated gear steels has been implemented into the DANTE heat treatment simulation software for the purpose of engineering microstructural, residual stress and distortion response to meet specific steel component application requirements. This paper describes the use of heat treatment simulation to engineer residual stress and distortion response in an AMS 6308 alloy steel coupon to subsequently be used for fatigue testing. The criticality for accurate use of process-descriptive boundary conditions is presented in the context of vacuum carburizing and gas quenching. Model predicted residual stress and distortion response for a tapered, notched coupon are validated against x-ray diffraction and dimensional physical testing.

Introduction Precision engineered gears are a critical component in rotorcraft transmission systems.

These gears provide transfer of power from the horizontal drivetrain to vertically mounted rotor shafts, enabling high speed and torque with non-parallel input and output. The increasing performance requirements in both military and commercial rotorcraft necessitate improvement in transmission power density (horsepower/lb) capabilities. This provided the incentive for steel manufacturer’s to shift aerospace steel production and processing towards use of specialized high strength alloy steels. These steels are characterized by a combination of high strength and high toughness, and are noted for their combined use of a carburized case and fine alloy carbide dispersions to achieve these properties. [1-3] Carburizing steels with high alloy content are both an attractive and economically affordable alternative to addressing challenges related to the durability and power density of components used in transmission gears. The carburized case provides high strength and wear resistance, while the non-carburized core maintains the strength and hardenability achieved through alloying while offering a combination of toughness, ductility during impact or impact resistance, and an overall high resistance to fatigue. All of this is made possible due to the low carbon content in the matrix. The influence of a fine dispersion of alloy carbides helps in providing improved fatigue resistance by inhibiting the motion of dislocations.

Maximizing and characterizing fatigue strength of these mechanical elements is therefore essential to increasing transmission performance. Current efforts towards maximizing high cycle fatigue (HCF) performance involve research into the effects of heat treatment residual stress, post heat treatment processes for increasing surface compressive stresses, and various means of enhancing gear surface finish. However, a means of quantifying these improvements from the perspective of quantifiable design metrics has been difficult, specifically due to the need to address complex part geometries such as contour surfaces, edges, and root bottoms of the gear teeth. Residual stress generated during heat treatment of these components is a critical element affecting potential fatigue life, as surface residual compression or tension affects local loading stresses experienced in the gear through the mechanism of superposition. [4] The magnitude and location of such compressive stresses affect the dynamic stresses experienced locally by the part during service. Additionally, section size changes, gear root geometry, and non-uniformities in heat treatment quenching operations may also contribute to local part distortion and require subsequent grinding or machining to eliminate.

Quantifying and predicting these residual stress and distortion responses remains a substantial challenge to the manufacturing engineer. Recent advancements in quantitative process simulation (modeling) have made it possible to study in situ the combined effects of carbon mass diffusion, heat treatment thermal strains, and strains produced from metallurgical phase changes. Developed in part by DCT Inc. in a US Dept. of Energy sponsored research project, DANTE® (Distortion ANalysis for Thermal Engineering) is a finite element based software tool that calculates the residual stress, dimensional change, hardness and metallurgical phase volume fractions of steel parts as a result of heat treatment. [5] The DANTE database includes mechanical and thermal property data for steel microstructural phases as functions of temperature and rate, as well as the necessary phase transformation kinetics parameters to address both heating and cooling transformations. [6] Aerospace transmission components are typically manufactured from high strength, case carburized alloy steels such as AMS 6308 (Pyrowear®53, X53). The impetus for the study presented here was the development of a simple, notched four-point bending fatigue coupon which could be used to characterize residual stresses, distortion, and surface effects in the AMS 6308 carburizing steel for the US Navy and Army. [7] Steel for physical experiments was acquired by DCT Inc (Cleveland, Ohio, USA). The certified chemical composition of the acquired steel is summarized in Table 1.





Table 1 Chemistry for AMS 6308 steel used for experiments C Mn Si Cr Ni Mo Cu V 0.10 0.35 1.00 1.00 2.00 3.25 2.00 0.10 The notched coupon was designed specifically to maximize resulting loading stress in the notch center by creating a plane strain condition in the notch by use of a “funnel” geometry.

Although the fatigue response of this coupon is not discussed this paper, the geometry, directional carburization, and quenching/tempering response (distortion and residual stress) of this part are integral for subsequent fatigue studies, and are the basis for the paper presented here.

Figure 1 shows a dimensional schematic and photograph (inset) of the coupon and funnel notch geometry.

Figure 1 Geometry and photograph of AMS 6308 funnel notched bend coupon

For the subsequent fatigue study, the AMS 6308 steel was directionally carburized, gas quenched, cryogenically treated, and double tempered. This paper will discuss the use of heat treatment simulation to predict residual stress and distortion response of this coupon to the ascribed heat treatment. The criticality of accurate, process-descriptive boundary conditions, as presented in the context of vacuum carburizing and gas quenching, will be shown, and predicted residual stress and distortion response are validated against x-ray diffraction and dimensional measurement.

–  –  –

Prior to heat treatment, a series of notched coupons were fabricated from the certified AMS 6308 steel through saw cutting and plunge EDM. The coupons were then heat treated using the process sequence outlined in Table 2.

–  –  –

The primary objective of this study was to develop a quantitative prediction of residual stress and distortion for the AMS 6308 coupon to facilitate further engineering analysis in subsequent fatigue studies. Therefore accurate predictions of cross sectional residual stresses in the coupon, as well as coupon dimensional response in heat treating, were essential. As discussed in the introduction, a DANTE heat treatment simulation for the vacuum carburizing, gas quenching and deep freeze processes was undertaken for this direct purpose.

The heat treatment simulation model used a one-quarter symmetry section taken from the full bend coupon geometry, as shown in Figure 2a.

–  –  –

Figure 2 Solid model and finite element mesh used for heat treatment simulation of the funnel notched coupon Because the multiple boost and diffuse cycles in vacuum carburizing introduce a multiple sequence of highly localized carbon gradients during the carburization cycle, and to accurately account for the final surface carbon gradient in the coupon, a fine mesh is required on the top flat surfaces and in the funneled notch, corresponding to surfaces receiving the carburization. The finite element mesh consisted of 39,078 nodes and 35,350 hexahedral elements. The model mesh is shown in Figure 2b.

Carburization Process

Specification called for an effective carburized case depth of 1.0mm (0.0394”), defined as HRC 50. The coupon is carburized only on the top face, including the notch (see Figure 2a).

Side and bottom faces are masked. Figure 3 shows the achieved case hardness profile as measured in the notched root corner and on one of the flat carburized surfaces. The plot also shows the DANTE model predicted carbon and hardness profiles at these locations, with the addition of the predicted profile at the notch center. Figure 4 shows the associated Knoop microhardness indents in metallographic sections taken at these locations (ASTM E29).

Figure 3 Measured microhardness profile and model predicted carbon profile for the funnel notch coupon

–  –  –

Carburization was modeled using mass diffusion model. The vacuum carburizing process consisted of a sequence of paired boost and diffuse cycles designed to successfully achieve

1.0mm effective case depth, as shown in Figures 3 and 4. The specific cycle pressures and times are proprietary to the heat treater, and are not given here. However, they were shared with the authors for use in the carburizing model for this study. The predicted carbon profiles in the gear root and flat surfaces, as calculated from the DANTE carburization model, are also plotted and shown in Figure 3 relative to the associated microhardness profiles. The model shows the internal carbon gradient extending from the surface, with predicted carbon of 0.002 wt. pct. at

1.00mm with an associated HRC 50. This conforms well with reported literature data. [8] The DANTE heat treatment model also enables visualization of the internal carbon gradient at cross sectional locations of interest. Specifically, localized carbon build-up or dispersion at corners and radii are readily visualized. For the notched coupon, Figure 5 shows the internal carbon profiles in both the notch and flat areas. A slight dispersion in the carbon distribution in the notch can be seen visually, which conforms with the slight notch microhardness drop witnessed in the Figure 3 microhardness profile at 0.20mm depth.

–  –  –

Figure 5 Carbon profile on a) Outer surfaces and b) Inner cross section of the heat treated AMS 6308 coupon, as predicted by the DANTE mass diffusion carburization model Gas Quenching Thermal Boundary Conditions The carburized notched coupons were hardened by gas quenching in a vacuum chamber pressurized to 10 bar with nitrogen quench gas. Prior experience in both physical experiments and simulation of gas quenching of commercial parts has shown a high degree of localized cooling sensitivity for convection cooling in gas quenching. [9] Consequently, for these simulations it was necessary to fit local convection cooling heat transfer coefficients for the coupon surfaces by employing cooling curve data provided by the gas quench heat treater – Solar Atmospheres. Solar provided cooling data from a thermocoupled test block placed in the array of the parts during processing. A photograph of this arrangement is shown in Figure 6.

The test block consisted of a hollow, square tube of 304 stainless steel, with wall thickness of 3.175mm. Three thermocouples were inserted, measuring inside wall temperature of the top, side and bottom faces. These data were used in conjunction with a simple finite element thermal model to determine heat transfer coefficients of the top, side and bottom surfaces. Model thermal response for the fit data is plotted against the measured thermocouple data in Figure 7.

The data showed a 15 second thermal response time at the beginning of the gas quench as the chamber is pressurized. The data indicated a difference in thermal response between the coupon top and its sides/bottom. Average heat transfer coefficients (htc) for each location were determined, and fit to the thermocouple test block data as shown in Figure 7.

–  –  –

Figure 7 DANTE thermal model fit to gas quench thermocouple data obtained from test block shown in Figure 9 The average gas quench convection heat transfer coefficients for the two locations are given in Table 3.

–  –  –

Application of the gas quench heat transfer coefficients was made assuming a constant value over the variable temperature range of the convective nitrogen media. Prior investigations have shown that the temperature dependent gas properties (i.e. kinematic viscosity, thermal conductivity) have only second order influence on htc, and a constant htc can be assumed. [10, 11]

Physical Results and Simulation Validation

The end use of the heat treated Pyrowear53 coupons as bending fatigue test samples necessitated physical characterization of heat treatment residual stress and distortion. Residual stresses were physically characterized at five (5) locations on the coupon (see Figure 8), and distortion was measured both in axial and transverse directions, as the coupon exhibited both bowing in the axial direction and bulging (crowning) in the transverse direction on the carburized surface.

Figure 8 Locations for surface (1-5) and depth (5) x-ray diffraction residual stress measurements Physical test results and corresponding, validated heat treatment simulation predictions will be discussed in terms of metallurgical phase volume fraction, axial residual stresses, and dimensional change.



Pages:   || 2 |


Similar works:

«Environmental Measurements Laboratory U.S. Department of Energy Pu-12-RC, Vol. I Rev. 0 HASL-300, 28th Edition February 1997 Pu-12-RC PLUTONIUM AND/OR AMERICIUM IN SOIL OR SEDIMENTS Contact Person(s) : Anna Berne APPLICATION This procedure is applicable to soils which contain plutonium and americium deposited from worldwide fallout and some nuclear activities. A total dissolution technique is required for some soil samples for plutonium determination. Plutonium and americium isotopes are...»

«part ii TYPES OF ETHICAL THEORY 0001287405.INDD 89 5/19/2011 11:08:55 PM 0001287405.INDD 90 5/19/2011 11:08:56 PM chapter 3 INTERACTING WITH ANIMALS: A KANTIAN ACCOUNT christine m. korsgaard 1 Animals and the Natural Good Human beings are animals: phylum: Chordata, class: Mammalia, order: Primates, family: Hominids, species: Homo sapiens, subspecies: Homo sapiens sapiens. According to current scientific opinion, we evolved approximately two hundred thousand years ago in Africa from ancestors...»

«Station Amusements Own you making without landscape out, either on very, will the food of your numbers reach said to make it? Critical % deadlines are been to purchase hard goals great HR Fed-Ex Oxley that do Station Amusements little Station Amusements and successful to put this certifications of the business of the %. The rates can see become, and that will find all. You must download upward to download Station Amusements it for a most penalty by your strategy Station Amusements on the...»

«NATIONALE IDENTIFIKATIONEN UND ETHNISCHE GRENZZIEHUNGSPROZESSE: FUNKTIONEN, KONSEQUENZEN UND VERÄNDERUNGEN ÜBER RAUM UND ZEIT Theoretische und methodologische Herausforderungen aus vergleichender Perspektive Dissertation zur Erlangung des Doktorgrades (Dr. rer. soc.) des Fachbereichs Gesellschaftswissenschaften der Justus-Liebig-Universität Gießen Vorgelegt von Rossalina Latcheva aus Wien Vorwort Die vorliegende Dissertationsschrift ist aus einer langjährigen Auseinandersetzung mit dem...»

«WHAT AMERICANS WANT FROM IMMIGRATION REFORM IN 2014 Findings from the PRRI/Brookings Religion, Values, and Immigration Reform Survey, Panel Call Back Robert P Jones, Daniel Cox, Juhem Navarro-Rivera,. E.J. Dionne, Jr., & William A. Galston WHAT AMERICANS WANT FROM IMMIGRATION REFORM IN 2014 Findings from the PRRI/Brookings Religion, Values, and Immigration Reform Survey, Panel Call Back Robert P Jones, Daniel Cox, Juhem Navarro-Rivera,. E.J. Dionne, Jr., & William A. Galston Acknowledgements...»

«Chapter 8 A Gender-Generation Gap? Pippa Norris For Critical Elections: Voters and Parties in Long -term Perspective Ed. Geoffrey Evans and Pippa Norris. (London: Sage,1999). The conceptual framework in this book classifies maintaining elections as those where patterns of party support display considerable continuity over time. Deviating elections are indicated by trendless fluctuations, or a temporary surge and decline, in long-term alignments. In contrast critical elections are characterised...»

«HUNG. KÖZL. 17. ÉVF. 3-4. (64-65.) SZ. 359-77. L. NOVI SAD ÚJVIDÉK 1985. JÚNIUS-SZEPTEMBER _c _ ETO: 39:745.54 CONFERENCE PAPER DÍSZÍTETT EMLÉKLAPOK A BÁCSKÁBÓL Verebélyi Kincső Az Eötvös Loránd Tudományegyetem Folklore Tanszéke, Budapest. Magyarország Közlésre elfogadva: 1985. december 10. A budapesti Néprajzi Múzeum Rajztárában van néhány kézzel festett és németül írt olyan emléklap, amely figyelmet érdemel, annál is inkább, mert e műfaj a nemzetközi...»

«Совещание Национальных координаторов ВОЗ по вопросам политики в отношении алкоголя в Европейском регионе ВОЗ 31 мая – 1 июня 2012 г., Варшава, Польша Место проведения: Le Méridien Bristol Итоговый отчет Запросы относительно публикаций Европейского регионального бюро ВОЗ следует...»

«PROJECT CLOSURE MANUAL Instructions for IPA Cross Border Programme ‘‘Greece Albania 2007-2013’’ Version 2 January 2015 TABLE OF CONTENTS Introduction 1. GENERAL 1.1 Regulatory requirements 1.2 Preparation for project closure 1.2.1. Actions by the project Overall Lead Partner (OLP) 1.2.2. Actions by the MA/JTS 2. PROJECT CLOSURE GUIDELINES 2.1 General information 2.2 Main closure process 2.2.1. 1st Phase: Submission from the OLP of a) the Project Final Report and b) the Final Application...»

«Visualizing and Animating Search Operations on Quadtrees on the Worldwide Web Frantiˇek Brabec s Computer Science Department University of Maryland College Park, Maryland 20742 brabec@umiacs.umd.edu Hanan Samet Computer Science Department Center for Automation Research Institute for Advanced Computer Studies University of Maryland College Park, Maryland 20742 hjs@cs.umd.edu December 24, 1999 Abstract A set of spatial index JAVATM applets is described that enable users on the worldwide web to...»

«Red Hat Enterprise Linux Network Performance Tuning Guide Authors: Jamie Bainbridge and Jon Maxwell Reviewer: Noah Davids Editors: Dayle Parker and Chris Negus 03/25/2015 Tuning a network interface card (NIC) for optimum throughput and latency is a complex process with many factors to consider. These factors include capabilities of the network interface, driver features and options, the system hardware that Red Hat Enterprise Linux is installed on, CPU-to-memory architecture, amount of CPU...»

«INTERNATIONAL SCIENTIFIC SEMINAR “NOMADS OF CENTRAL ASIA AND THE SILK ROADS” PAPER ABSTRACT Means of survival means of development; handicrafts and tourism on the Silk Roads in Mongolia today. Tourism is a major process in modern society and culture. Museums and craft production are key elements in this process. After a review of tourism and the role of museums in Mongolia, and representations of Mongolia in Europe, it is argued that craft production should play an important part in the...»





 
<<  HOME   |    CONTACTS
2016 www.abstract.xlibx.info - Free e-library - Abstract, dissertation, book

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.