This work has had the purpose to examine defects and the cause of crack growth during long term storage in rolled bars. The bars in question are rolled and stored at Ovako in Hofors. The problem is approached by theoretically examining the possibilities of hydrogen damage, room temperature creep and naturally occurring porosity. Seven bars of three steel type groups and with different rolling dates were selected and examined for defects with ultrasonic testing. Four of the bars show defects. Specimens containing the defects are cut out and grinded so that the defects can be examined in the scanning electron microscope and analyzed with energy dispersive x-ray spectroscopy. One of the specimens shows some form of cavity containing iron carbide. One of the others shows a surface with inclusions. The remaining two both shows inclusions and cracks that seem to originate from inclusions. The theoretical studies show that there are two types of hydrogen damage that could be in effect in the studied case and also that room temperature creep could facilitate but not cause crack growth and that porosity could serve as crack initiation points. Steps should be taken to minimize the problems mentioned above when they are suspected causes. Further examination of inclusions in different steel types should be made if considered a problem. / Detta arbete har haft syftet att undersöka defekter och orsaken till spricktillväxt under långtidsförvaring av spårvalsade stänger. Stängerna i undersökningen är valsade och lagrade hos Ovako i Hofors. Problemet är angripet genom att teoretiskt undersöka risken för väteskador, kryp i rumstemperatur och naturligt förekommande porositet. Sju stänger indelade i tre stålsortsgrupper och med olika valsningsdatum valdes ut och undersöktes med ultraljud. Fyra av stängerna visade på defekter. Prov innehållande dessa defekter blev utkapade och slipade så att defekterna kan bli undersökta i svepelektronmikroskop och analyserade med energidispersiv röntgenanalys. Ett av proverna visar på ett hålrum innehållande en järnkarbid. Ett av de andra proverna visar på en yta med inneslutningar. De två återstående proverna visar på inneslutningar och sprickor som verkar utgå från inneslutningar. De teoretiska studierna visar att det finns två typer av väteskador som är aktuella i det studerade fallet och även att kryp vid rumstemperatur kan underlätta spricktillväxt men inte orsaka den och att porositet kan tjäna som sprickinitieringsställen. Åtgärder för att minimera de nämnda problemen bör tas i fall där de är misstänkta orsaker. Ytterligare undersökningar av inneslutningsbilden bör göras om inneslutningar anses vara ett problem.

Hydrogen embrittlement is a problem that offers challenges both to technology and to the theory of metallurgy. In the presence of a hydrogen rich environment, applications such as rolling bearings display a significant decrease in alloy strength and accelerated failure due to rolling contact fatigue. In spite of these problems being well recognised, there is little understanding as to which mechanisms are present in hydrogen induced bearing failure. The objective of this thesis are twofold. First, a novel alloy combining the excellent hardness of bearing steels, and resistance to hydrogen embrittlement, is proposed. Second, a new technique to identify the nature of hydrogen embrittlement in bearing steels is suggested. The new alloy was a successful result of computer aided alloy design; thermodynamic and kinetic modelling were employed to design a composition and heat treatment combining (1) fine cementite providing a strong and ductile microstructure, and (2) nano-sized vanadium carbide precipitates acting as hydrogen traps. A novel technique is proposed to visualise the migration of hydrogen to indentation-induced cracks. The observations employing this technique strongly suggest that hydrogen enhanced localised plasticity prevails in bearing steels. While proposing a hydrogen tolerant bearing steel grade, and a new technique to visualize hydrogen damage, this thesis is expected to aid in increasing the reliability of bearings operating in hydrogen rich environments.