That's a common problem with older 031s. The muffler mounting screws loosen, operator doesn't notice till the crack(s) are present. Locktite those screws in when you re-install your brazed muffler.
Some of the models that are susceptable to this malady are: 031, 032, 041, 045, 051. Probably others too, but those are one's I'm familiar with this issue. Stihl updated the screws in later models (torx w/locking head) that helps, but I stihl locktite all muffler installs regardless of vintage. On 031s I'd locktite handle, AV mounts (if an AV model) and recoil screws (blue/medium locktite here). I use the harder stuff on the muffler. Good luck with your 031, good older saw. I have two of them in my stable.