The　effect　of　microstructure　as　a　function　of　welding　cooling　time　(t8/5)　from　800　to　500　on　the　impact　toughness　of　coarse　grained　heat　affected　zone　(CGHAZ)　of　deposited　metal　of　metal　cored　wire　E120C-K4　was　investigated　by　welding　thermal　simulation.　The　results　showed　that　the　microstructure　of　CGHAZ　was　mainly　composed　of　degenerate　upper　bainite　(DUB)　granular　bainite　(GB)　and　Acicular　ferrite(AF)　at　t8/5　from　6　s　to　12　s,　forming　the　interlace　multiphase　microstructure,　then　the　optimal　impact　toughness　was　obtained.　The　microstructure　of　CGHAZ　formed　with　granular　bainite　(GB)　and　Acicular　ferrite(AF)　and　the　impact　toughness　decreased　when　t8/5　from　30　s　to　120　s.　The　absorbed　energy　of　CGHAZ　was　only　24　J　at　-40　at　t8/5　of　120　s,　the　worst　impact　toughness.　The　key　of　improving　impact　toughness　at　t8/5　from　6　s　to　12　s　was:　forming　the　interlace　multiphase　microstructure;　refining　grains;　more　high-angle　grain　boundaries　per　unit　distance.　©　2019,　Editorial　Board　of　Transactions　of　the　China　Welding　Institution,　Magazine　Agency　Welding.　All　right　reserved.